Lesson 65: CSS Variables · The var() Function, Overriding, JavaScript, and Media Queries

Phase 1 of 8

Lesson Introduction

Have you ever built a webpage and needed to use the same colour · say, a brand blue · in twenty different places? And then the client says "change it to green." Suddenly you are hunting through hundreds of lines of CSS making the same edit over and over.

CSS Variables (also called CSS Custom Properties) solve this problem completely. You store a value once, give it a name, and reuse that name everywhere. When you want to change the value, you change it in exactly one place and the whole page updates instantly.

In this lesson you will learn:

What a CSS variable is and how to declare one
How to use the var() function to insert variable values
How global and local scope work
How to override variables for specific sections of a page
How to read and change CSS variables with JavaScript
How to use CSS variables inside media queries to build responsive designs
How to apply all of this in a real mini-project

No prior knowledge of CSS variables is required. If you know basic CSS (selectors, properties, values) you are ready.

✏️ Your Task

Practise what you just learned about Lesson Introduction. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 2 of 8

Prerequisite Concepts

Before we dive into CSS variables, let us quickly review two concepts you need to understand first.

What is a CSS Property?

A CSS property is an instruction you give to the browser about how something should look. For example:

css

p {
  color: red;
  font-size: 16px;
}

color is a property
red is its value

What is a Selector?

A selector targets which HTML elements you want to style. p targets all paragraph elements, .note targets all elements with class note, and so on.

Now you are ready!

✏️ Your Task

Practise what you just learned about Prerequisite Concepts. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 3 of 8

Part 1 · CSS Variables and the var() Function

What is a CSS Variable?

A CSS variable is a named storage container for a value you want to reuse. Think of it like a labelled jar in your kitchen. Instead of remembering that your favourite spice is "reddish-orange powder", you label the jar "paprika". Now whenever you need it you just say "grab the paprika jar."

In CSS:

Instead of writing #1e90ff in twenty places, you store it in a variable named · brand-color.
Whenever you need that colour, you write var( · brand-color) and CSS grabs the value from the jar.

Why Do CSS Variables Exist? What Problem Do They Solve?

Without variables, large stylesheets suffer from three problems:

Repetition · the same colour code or font size written 50 times
Hard maintenance · change one value means editing 50 places (and missing some)
Low readability · #1e90ff means nothing; · primary-color is self-documenting

CSS variables solve all three at once.

Naming Rules

A CSS variable name must:

Begin with exactly two dashes: ·
Be followed by a descriptive name using letters, numbers, and hyphens
Be case-sensitive · · MyColor and · mycolor are two different variables

css

/* ✅ Valid variable names */
--primary-color: blue;
--font-size-large: 24px;
--spacing-unit: 8px;
--header-bg: #333333;

/* ❌ Invalid — missing the double dash */
-primary-color: blue;    /* wrong */
primary-color: blue;     /* wrong */

Declaring a Variable

You declare a variable inside a CSS selector block, like any other property:

css

selector {
  --variable-name: value;
}

There are two scopes · global and local. Let us learn them one at a time.

Global Variables · The `:root` Selector

A global variable is available to every single element on the page. To make a variable global, declare it inside the :root selector.

What is :root? :root is a special CSS selector that matches the very top of your HTML document · the <html> element. Because every other element on the page is a child of <html>, any variable declared in :root is visible everywhere on the page.

Analogy: :root is like the front lobby of a building. Anything placed in the lobby can be seen by everyone in the building.

css

/* Declaring two global variables */
:root {
  --primary-color: #1e90ff;    /* a nice blue colour */
  --text-color: #ffffff;       /* white */
}

Local Variables · Inside a Regular Selector

A local variable is only available to the selector where it is declared, and to any elements nested inside it. Elements outside that selector cannot see it.

css

.card {
  --card-bg: lightyellow;    /* local — only .card and its children can use this */
}

Analogy: A local variable is like a note pinned to a specific desk. Only the person sitting at that desk (and anyone they share it with) can read it.

The `var()` Function · Using Your Variables

Once you have declared a variable, you use the var() function to insert its value anywhere in your CSS.

Syntax:

code

var(--variable-name, fallback-value)

Part	What it means
`· variable-name`	Required. The name of the variable you want to use. Must start with `·` .
`fallback-value`	Optional. If the variable is not found, CSS uses this value instead.

Simple Example 1 · Using a colour variable:

css

:root {
  --brand-color: tomato;
}

h1 {
  color: var(--brand-color);   /* h1 text becomes tomato red */
}

Expected Result: The <h1> heading will display in tomato red.

Simple Example 2 · Using a fallback value:

css

:root {
  /* --missing-var is NOT declared */
}

p {
  color: var(--missing-var, black);   /* fallback kicks in → text is black */
}

Expected Result: Because · missing-var does not exist, CSS uses the fallback black instead.

💡 Tip: Always add a fallback for critical properties, especially when the variable might not always be defined.

Full Global Variable Example

Here is a complete realistic example. Notice how · primary-bg-color and · primary-color are declared once but used in four different places.

css

:root {
  --primary-bg-color: #1e90ff;   /* dodger blue background colour */
  --primary-color: #ffffff;      /* white text colour */
}

body {
  background-color: var(--primary-bg-color);   /* body background becomes blue */
}

.container {
  color: var(--primary-bg-color);              /* text inside container is blue */
  background-color: var(--primary-color);      /* container background is white */
  padding: 15px;
}

.container h2 {
  border-bottom: 2px solid var(--primary-bg-color);   /* blue underline on h2 */
}

.container .note {
  border: 1px solid var(--primary-bg-color);          /* blue border on .note */
  padding: 10px;
}

Expected Visual Result:

The whole page body has a blue background
The container has a white background with blue text
The h2 heading has a blue underline
The .note box has a blue border
All blue values come from the single variable · primary-bg-color

Now for the magic · changing the entire look with 2 lines:

css

:root {
  --primary-bg-color: #8FBC8F;   /* ← changed from blue to soft green */
  --primary-color: #FFFAF0;      /* ← changed from white to floral white */
}

Without touching the body, .container, h2, or .note rules at all · every colour on the page updates automatically. That is the real power of CSS variables.

🤔 Think About This: What would have to change if you had NOT used variables, and needed to update the colour in all four selectors manually?

Advantages of CSS Variables · Summary

Using var() gives you three key advantages:

Easier maintenance · change one value, everything updates
Easier to read · · primary-bg-color is far more meaningful than #1e90ff
Easier to update colours across an entire page with minimal effort

✏️ Your Task

Practise what you just learned about CSS Variables and the var() Function. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 4 of 8

Part 2 · Overriding CSS Variables

What Does "Overriding a Variable" Mean?

Overriding means giving a variable a different value in a specific context, while keeping the original global value everywhere else.

Think of it this way: your company's brand colour is blue globally across the entire website. But one special "danger" section of the page needs to use red. You can override · primary-color just for that section, setting it to red, without changing the global blue definition.

This works because of a concept called the cascade and inheritance.

How Variable Overriding Works · The Cascade Rule

When CSS looks up a variable inside var(), it travels up the DOM tree from the current element, searching for the nearest declaration of that variable. It uses whichever declaration it finds first · either on the element itself, on a parent, or eventually reaching :root.

This means:

A variable declared closer to an element wins over the global :root version
The more specific (closer) declaration overrides the more general one

Example 1 · Simple Override

css

/* STEP 1: Declare globally in :root */
:root {
  --button-color: #1e90ff;   /* default blue */
}

/* STEP 2: All buttons use the global variable */
button {
  background-color: var(--button-color);
  color: white;
  padding: 10px 20px;
}

/* STEP 3: Override ONLY for buttons inside .danger-zone */
.danger-zone {
  --button-color: #ff4444;   /* local override — red */
}

Expected Result:

Buttons everywhere on the page → blue background
Buttons inside .danger-zone → red background (because the local · button-color overrides the global one)

Example 2 · Overriding for a Specific Element Class

Imagine a card design where most cards use a white background, but "featured" cards use gold.

css

:root {
  --card-bg: white;           /* global default */
  --card-border: #cccccc;     /* grey border globally */
}

.card {
  background-color: var(--card-bg);
  border: 2px solid var(--card-border);
  padding: 20px;
  margin: 10px;
}

/* Override: featured cards get different colours */
.card.featured {
  --card-bg: gold;            /* local override */
  --card-border: orange;      /* local override */
}

Expected Result:

.card → white background, grey border
.card.featured → gold background, orange border
No changes to the .card rule itself were needed

Example 3 · Using Local Variables for Component Theming

This pattern is extremely common in professional web development. Each component declares its own local variables that can be customised without affecting other components.

css

:root {
  --spacing: 10px;
  --text-color: #333;
}

/* Navigation uses its own overrides */
nav {
  --text-color: white;      /* nav text is white */
  --spacing: 20px;          /* nav spacing is bigger */
  color: var(--text-color);
  padding: var(--spacing);
  background-color: #222;
}

/* Footer uses different overrides */
footer {
  --text-color: #888;       /* footer text is grey */
  color: var(--text-color);
  padding: var(--spacing);  /* inherits global 10px spacing */
}

/* Main content uses the global defaults */
main {
  color: var(--text-color);   /* gets global #333 */
  padding: var(--spacing);    /* gets global 10px */
}

Expected Result:

nav → white text, 20px padding
footer → grey text, 10px padding (inherits global · spacing because footer didn't override it)
main → dark grey text (#333), 10px padding

🤔 Think About This: What happens if you add · text-color: red inside main p {}? Which elements would become red?

The Override Hierarchy · Visualised

code

:root { --color: blue }         ← global fallback (used by everything by default)
   └── .section { --color: green }  ← overrides for .section and all its children
         └── .highlight { --color: red }  ← overrides again, only for .highlight

An element inside .highlight will see red. An element inside .section but NOT .highlight will see green. Everything else sees blue.

Common Mistake: Forgetting Variable Scope

css

/* ❌ MISTAKE: Declaring a variable in a child, trying to use it in a parent */
.card-body {
  --card-accent: purple;
}

.card {
  border-color: var(--card-accent);  /* ❌ This won't work! .card cannot see .card-body's variable */
}

Why it fails: Variables only flow downward through child elements, not upward to parents.

Fix: Move the declaration to the parent (.card) or to :root.

css

/* ✅ CORRECT */
.card {
  --card-accent: purple;
  border-color: var(--card-accent);  /* ✅ Works — declared in the same element */
}

✏️ Your Task

Practise what you just learned about Overriding CSS Variables. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 5 of 8

Part 3 · CSS Variables and JavaScript

Why Use JavaScript with CSS Variables?

CSS by itself is static · it applies styles when the page loads and those styles stay the same. But real websites are dynamic. Users click buttons, toggle dark mode, adjust font sizes, or interact with controls. JavaScript lets you:

Read the current value of a CSS variable
Change a CSS variable's value at any time
React to user actions and update the entire page's look instantly

Because CSS variables are part of the DOM (Document Object Model · the live, interactive version of your webpage), JavaScript can see and modify them directly.

Key JavaScript Methods for CSS Variables

You need two browser built-in methods:

Method	What it does
`getPropertyValue(' · name')`	Reads the current value of a CSS variable
`setProperty(' · name', 'value')`	Sets or changes a CSS variable's value

These methods work on a style object. To access :root variables from JavaScript, you use:

javascript

document.documentElement.style

document.documentElement refers to the <html> element, which is the same element :root targets in CSS.

Reading a CSS Variable with JavaScript

Step-by-step breakdown:

javascript

// Step 1: Get a reference to the root element (the <html> element)
var root = document.querySelector(':root');

// Step 2: Get the computed styles of the root element
var rootStyles = getComputedStyle(root);

// Step 3: Read the value of a CSS variable
var value = rootStyles.getPropertyValue('--primary-color');

// Step 4: Do something with the value (e.g. show it)
alert(value);   // shows: " #1e90ff" (note there may be a leading space)

Line-by-line explanation:

document.querySelector(':root') · finds the root HTML element
getComputedStyle(root) · gets ALL the computed (final calculated) CSS styles applied to the root
.getPropertyValue(' · primary-color') · extracts just the value of · primary-color

💡 Note: getPropertyValue returns the value as a string. It may include leading or trailing spaces, so use .trim() if you need a clean value: value.trim()

Changing a CSS Variable with JavaScript

javascript

// Step 1: Get the root element
var root = document.querySelector(':root');

// Step 2: Set a new value for a CSS variable
root.style.setProperty('--primary-color', 'coral');

What happens: Every single element on the page that uses var( · primary-color) instantly updates to coral · no page reload needed.

Complete Interactive Example · Colour Switcher

Here is a complete working example. When the user clicks a button, JavaScript changes the CSS variable and the whole page updates instantly.

HTML:

html

<!DOCTYPE html>
<html>
<head>
  <style>
    :root {
      --page-bg: #1e90ff;       /* global background: blue */
      --page-text: #ffffff;     /* global text: white */
    }

    body {
      background-color: var(--page-bg);
      color: var(--page-text);
      font-family: Arial, sans-serif;
      padding: 30px;
      transition: background-color 0.5s;  /* smooth colour change */
    }

    button {
      padding: 10px 20px;
      margin: 5px;
      cursor: pointer;
      font-size: 16px;
    }
  </style>
</head>
<body>
  <h1>CSS Variables Demo</h1>
  <p>Click a button to change the page colour scheme.</p>

  <button onclick="setTheme('blue')">Blue Theme</button>
  <button onclick="setTheme('green')">Green Theme</button>
  <button onclick="setTheme('dark')">Dark Theme</button>

  <script>
    function setTheme(theme) {
      // Get the root element
      var root = document.querySelector(':root');

      // Set variables based on chosen theme
      if (theme === 'blue') {
        root.style.setProperty('--page-bg', '#1e90ff');
        root.style.setProperty('--page-text', '#ffffff');
      } else if (theme === 'green') {
        root.style.setProperty('--page-bg', '#2e8b57');
        root.style.setProperty('--page-text', '#f0fff0');
      } else if (theme === 'dark') {
        root.style.setProperty('--page-bg', '#1a1a2e');
        root.style.setProperty('--page-text', '#e0e0e0');
      }
    }
  </script>
</body>
</html>

Expected Behaviour:

Page starts with a blue background and white text
Click "Green Theme" → background instantly changes to dark green, text to mint
Click "Dark Theme" → background changes to near-black, text to light grey
Click "Blue Theme" → returns to the original blue

Why this is powerful: All the colour rules (body, any headings, paragraphs, borders, etc.) could all reference the same two variables. Changing just those two variables from JavaScript updates the entire page in one moment.

Reading and Displaying a Variable Value

html

<button onclick="showColor()">Show Current Background Color</button>

<script>
  function showColor() {
    var root = document.querySelector(':root');
    var styles = getComputedStyle(root);
    var color = styles.getPropertyValue('--page-bg').trim();
    alert('Current background color variable: ' + color);
  }
</script>

Expected Behaviour: Clicking the button shows an alert like: Current background color variable: #1e90ff

🤔 Think About This: If you call setProperty to change · page-bg, then call getPropertyValue to read it · will you get the new value or the old one? Try it!

✏️ Your Task

Practise what you just learned about CSS Variables and JavaScript. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 6 of 8

Part 4 · CSS Variables in Media Queries

What is a Media Query? (Quick Prerequisite)

A media query is a CSS technique that lets you apply different styles depending on the screen size or device type. For example:

css

/* Default styles for all screens */
p { font-size: 16px; }

/* Extra styles ONLY on screens narrower than 600px (mobile) */
@media screen and (max-width: 600px) {
  p { font-size: 14px; }
}

This is how websites adapt their layout for phones, tablets, and desktops.

Why Use CSS Variables Inside Media Queries?

Without variables, making something responsive means repeating and changing dozens of individual property values inside every media query. With variables, you just redefine the variables at the breakpoint · and everything that uses those variables updates automatically.

The pattern:

Define your variables globally in :root (for large screens)
Inside a media query, override those variables with new values (for small screens)
All your element rules stay unchanged · they still just say var( · name)

Simple Example 1 · Responsive Font Sizing

css

/* ✅ GLOBAL: Large screen font sizes */
:root {
  --heading-size: 48px;
  --body-size: 18px;
}

h1 { font-size: var(--heading-size); }
p  { font-size: var(--body-size); }

/* 📱 MOBILE: Override the variables for small screens */
@media screen and (max-width: 600px) {
  :root {
    --heading-size: 28px;   /* smaller heading for phones */
    --body-size: 14px;      /* smaller body text for phones */
  }
}

Expected Result:

On desktop (wider than 600px): h1 = 48px, p = 18px
On mobile (600px or narrower): h1 = 28px, p = 14px
The h1 and p CSS rules themselves never changed · only the variables changed

Simple Example 2 · Responsive Spacing

css

:root {
  --page-padding: 40px;      /* comfortable spacing on large screens */
  --card-gap: 20px;
}

.page-wrapper {
  padding: var(--page-padding);
}

.card-grid {
  gap: var(--card-gap);
}

/* Override for small screens */
@media screen and (max-width: 768px) {
  :root {
    --page-padding: 15px;    /* tighter on mobile */
    --card-gap: 10px;
  }
}

Expected Result:

Large screens: wrapper has 40px padding, grid has 20px gap
Mobile screens: wrapper has 15px padding, grid has 10px gap · all from changing two variable values

Full Responsive Colour Theme Example

This is a practical, real-world pattern where variables control both layout AND colours across breakpoints:

css

/* ===== GLOBAL VARIABLES (Desktop defaults) ===== */
:root {
  /* Colors */
  --bg-color: #f5f5f5;
  --text-color: #333333;
  --accent-color: #1e90ff;
  --card-bg: #ffffff;

  /* Layout */
  --container-width: 1100px;
  --sidebar-width: 260px;
  --content-padding: 30px;
  --font-size-base: 18px;
}

/* ===== ELEMENT RULES (unchanged across breakpoints) ===== */
body {
  background-color: var(--bg-color);
  color: var(--text-color);
  font-size: var(--font-size-base);
}

.container {
  max-width: var(--container-width);
  padding: var(--content-padding);
}

.sidebar {
  width: var(--sidebar-width);
  background-color: var(--card-bg);
}

.accent-button {
  background-color: var(--accent-color);
  color: white;
  padding: 10px 20px;
}

/* ===== TABLET breakpoint (768px) ===== */
@media screen and (max-width: 768px) {
  :root {
    --sidebar-width: 200px;       /* narrower sidebar on tablet */
    --content-padding: 20px;      /* less padding */
    --font-size-base: 16px;       /* slightly smaller font */
  }
}

/* ===== MOBILE breakpoint (480px) ===== */
@media screen and (max-width: 480px) {
  :root {
    --sidebar-width: 100%;        /* full-width sidebar stacks on mobile */
    --content-padding: 12px;      /* minimal padding */
    --font-size-base: 14px;       /* compact font */
    --bg-color: #ffffff;          /* cleaner white background on mobile */
  }
}

Expected Result:

Desktop: wide container, 260px sidebar, 30px padding, 18px font, grey background
Tablet: 200px sidebar, 20px padding, 16px font
Mobile: full-width sidebar, 12px padding, 14px font, white background

The key insight: The .sidebar, body, .container, and .accent-button rules never changed. Only the variable values at the top changed at each breakpoint.

💡 Real-World Tip: This pattern · variables in :root, overrides in media queries · is how professional CSS design systems like those used at large companies are built. Change the variables, and the entire system adapts.

✏️ Your Task

Practise what you just learned about CSS Variables in Media Queries. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 7 of 8

Part 5 · Guided Practice Exercises

🎯 Your Challenge

Exercise 1 · Brand Colour System (Beginner)

Objective: Practice declaring and using global CSS variables.

Scenario: You are styling a simple webpage for a fictional company called "LeafCo" whose brand colour is #2e8b57 (sea green).

Your Task:

Declare two global variables in :root:

- · brand-color set to #2e8b57 - · brand-text set to #ffffff

Use var( · brand-color) as the background-color for the header
Use var( · brand-text) as the color for header h1
Use var( · brand-color) as the border-left colour for all blockquote elements (4px solid)
Use var( · brand-color) as the text color for all a links

Starter HTML:

✏️ Task

Expected Behaviour: - Starts in light mode (white background) - Clicking the button switches to dark mode (dark blue tones) - Clicking again returns to light mode ·

html

<!DOCTYPE html>
<html>
<head>
  <style>
    /* YOUR CSS HERE */
  </style>
</head>
<body>
  <header>
    <h1>Welcome to LeafCo</h1>
  </header>
  <main>
    <blockquote>We grow the best plants in the city.</blockquote>
    <p>Visit our <a href="#">online store</a> today.</p>
  </main>
</body>
</html>

Expected Result:

Header has a green background with white heading
The blockquote has a green left border
The link text is green
Changing · brand-color to #8B0000 (dark red) updates all four elements at once

Self-Check Questions:

Where did you declare the variables? (Should be inside :root)
Can you change the entire colour scheme by editing just one line?

Exercise 2 · Section Colour Override (Intermediate)

Objective: Practice local overrides of global variables.

Scenario: A blog website has a default card style (white background, blue accent). But the "Featured Posts" section should have a gold accent.

Your Task:

css

:root {
  --card-bg: #ffffff;
  --card-accent: #1e90ff;   /* blue */
  --card-border-radius: 8px;
}

.card {
  background-color: var(--card-bg);
  border-left: 5px solid var(--card-accent);
  border-radius: var(--card-border-radius);
  padding: 20px;
  margin-bottom: 15px;
}

/* TODO: Add a rule for .featured-section that overrides
   --card-accent to gold (#FFD700)
   and --card-bg to #fffde7 (light yellow) */

Expected Result:

Regular .card → white background, blue left border
.card inside .featured-section → light yellow background, gold left border

Hints:

Override the variables on .featured-section, not on .card itself
You do NOT need to duplicate the .card rule

Exercise 3 · JavaScript Theme Toggle (Intermediate)

Objective: Use JavaScript to read and change CSS variables.

Scenario: Build a simple "light mode / dark mode" toggle button.

Starter Code:

html

<!DOCTYPE html>
<html>
<head>
  <style>
    :root {
      --bg: #ffffff;
      --text: #333333;
      --card-bg: #f0f0f0;
    }

    body {
      background-color: var(--bg);
      color: var(--text);
      font-family: Arial, sans-serif;
      padding: 20px;
      transition: all 0.3s;
    }

    .card {
      background-color: var(--card-bg);
      padding: 20px;
      border-radius: 8px;
      margin-top: 20px;
    }
  </style>
</head>
<body>
  <button id="toggle-btn" onclick="toggleMode()">Switch to Dark Mode</button>

  <div class="card">
    <h2>Sample Card</h2>
    <p>This card will change colours when you toggle the mode.</p>
  </div>

  <script>
    var isDark = false;

    function toggleMode() {
      var root = document.querySelector(':root');
      var btn = document.getElementById('toggle-btn');

      if (!isDark) {
        // TODO: Set dark mode variables
        // --bg should become #1a1a2e
        // --text should become #e0e0e0
        // --card-bg should become #16213e
        btn.textContent = 'Switch to Light Mode';
        isDark = true;
      } else {
        // TODO: Set light mode variables (back to original values)
        btn.textContent = 'Switch to Dark Mode';
        isDark = false;
      }
    }
  </script>
</body>
</html>

Your Task: Fill in the TODO comments with the correct root.style.setProperty(...) calls.

Expected Behaviour:

Starts in light mode (white background)
Clicking the button switches to dark mode (dark blue tones)
Clicking again returns to light mode

Phase 8 of 8

Part 7 · Common Beginner Mistakes

Mistake 1: Forgetting the Double Dash

css

/* ❌ WRONG — only one dash */
:root {
  -my-color: blue;
}

/* ✅ CORRECT — two dashes */
:root {
  --my-color: blue;
}

Why: CSS custom property names MUST start with · . A single dash is valid CSS but means something else (it could be a vendor prefix).

Mistake 2: Forgetting `var()` When Using the Variable

css

/* ❌ WRONG — writing the variable name directly */
h1 {
  color: --my-color;    /* CSS does NOT do this */
}

/* ✅ CORRECT — using the var() function */
h1 {
  color: var(--my-color);
}

Why: CSS variables are not automatically expanded. You must explicitly call var() to insert the value.

Mistake 3: Trying to Use a Variable Before Declaring It

css

/* ❌ WRONG — using it before declaring it */
h1 {
  color: var(--brand);
}

:root {
  --brand: purple;    /* declared after use */
}

Result: This actually works in CSS because :root is processed first regardless of order, but it is bad practice and confusing. Always declare variables in :root at the TOP of your stylesheet.

Mistake 4: Using a Variable in the Wrong Scope

css

/* ❌ MISTAKE — variable declared in child, used in sibling or parent */
.card-footer {
  --footer-bg: lightgrey;
}

.card-header {
  background-color: var(--footer-bg);   /* ❌ .card-header cannot see .card-footer's variable */
}

Fix: Move the variable to the shared parent element, or to :root.

Mistake 5: Wrong JavaScript Method Name

javascript

/* ❌ WRONG */
root.setProperty('--color', 'red');          /* not a method on the element */
root.style.getProperty('--color');           /* wrong name — it's getPropertyValue */

/* ✅ CORRECT */
root.style.setProperty('--color', 'red');
getComputedStyle(root).getPropertyValue('--color');

Mistake 6: Forgetting getComputedStyle When Reading Variables

javascript

/* ❌ WRONG — this reads inline styles only, not :root CSS variables */
var color = root.style.getPropertyValue('--color');   /* likely returns empty string */

/* ✅ CORRECT — use getComputedStyle to read all applied styles */
var color = getComputedStyle(root).getPropertyValue('--color');

Why: root.style only contains inline styles. CSS variables declared in a stylesheet are only visible through getComputedStyle.

Mistake 7: Media Query Override Not in `:root`

css

/* ❌ WRONG — variable declared on body, but elements inherit from :root */
@media (max-width: 600px) {
  body {
    --font-size: 14px;    /* ⚠️ This might not reach all elements */
  }
}

/* ✅ CORRECT — override in :root for reliable global reach */
@media (max-width: 600px) {
  :root {
    --font-size: 14px;    /* ✅ All elements can see this */
  }
}

✏️ Your Task

· ### Mistake 4: Using a Variable in the Wrong Scope `css /* ❌ MISTAKE · variable declared in child, used in sibling or parent */ .card-footer { · footer-bg: lightgrey; } .card-header { background-color: var( · footer-bg); /* ❌ .card-header cannot see .card-footer's variable */ } ` Fix: Move the variable to the shared parent element, or to :root. ·

🏗️ Build It — Mini Project

Project Overview

You will build a simple single-page website for a fictional café called "The Bean Scene". The page will:

Use CSS variables for all colours, fonts, and spacing
Override variables for different sections (hero, menu, footer)
Include a JavaScript theme switcher (Day Mode / Night Mode)
Update its layout variables for mobile screens via media queries

Stage 1 · Setup: Define Your Design Tokens

In CSS, "design tokens" are just what professionals call the set of global variables that define your visual design system.

starter.html

:root {
  /* ===== COLOURS ===== */
  --color-primary: #6b4226;       /* coffee brown */
  --color-secondary: #d4a96a;     /* warm caramel */
  --color-bg: #fdf6ec;            /* creamy off-white */
  --color-text: #3e2723;          /* dark espresso */
  --color-text-light: #ffffff;
  --color-card-bg: #ffffff;
  --color-border: #e0c9a6;

  /* ===== TYPOGRAPHY ===== */
  --font-size-base: 16px;
  --font-size-large: 24px;
  --font-size-hero: 48px;

  /* ===== SPACING ===== */
  --padding-section: 60px 40px;
  --padding-card: 25px;
  --gap-cards: 20px;

  /* ===== SHAPES ===== */
  --border-radius: 10px;
}

Lesson 65 complete! 🎉

You covered:

✅ Lesson Introduction
✅ Prerequisite Concepts
✅ Part 1 · CSS Variables and the var() Function
✅ Part 2 · Overriding CSS Variables
✅ Part 3 · CSS Variables and JavaScript
✅ Part 4 · CSS Variables in Media Queries
✅ Part 5 · Guided Practice Exercises
✅ Part 7 · Common Beginner Mistakes

← Lesson 64 Lesson 66 →

CSS Variables · The var() Function, Overriding, JavaScript, and Media Queries

👋 Welcome to Lesson 65

What is a CSS Property?

What is a Selector?

What is a CSS Variable?

Why Do CSS Variables Exist? What Problem Do They Solve?

Naming Rules

Declaring a Variable

Global Variables · The :root Selector

Local Variables · Inside a Regular Selector

The var() Function · Using Your Variables

Full Global Variable Example

Advantages of CSS Variables · Summary

What Does "Overriding a Variable" Mean?

How Variable Overriding Works · The Cascade Rule

Example 1 · Simple Override

Example 2 · Overriding for a Specific Element Class

Example 3 · Using Local Variables for Component Theming

The Override Hierarchy · Visualised

Common Mistake: Forgetting Variable Scope

Why Use JavaScript with CSS Variables?

Key JavaScript Methods for CSS Variables

Reading a CSS Variable with JavaScript

Changing a CSS Variable with JavaScript

Complete Interactive Example · Colour Switcher

Reading and Displaying a Variable Value

What is a Media Query? (Quick Prerequisite)

Why Use CSS Variables Inside Media Queries?

Simple Example 1 · Responsive Font Sizing

Simple Example 2 · Responsive Spacing

Full Responsive Colour Theme Example

Exercise 1 · Brand Colour System (Beginner)

Exercise 2 · Section Colour Override (Intermediate)

Exercise 3 · JavaScript Theme Toggle (Intermediate)

Mistake 1: Forgetting the Double Dash

Mistake 2: Forgetting var() When Using the Variable

Mistake 3: Trying to Use a Variable Before Declaring It

Mistake 4: Using a Variable in the Wrong Scope

Mistake 5: Wrong JavaScript Method Name

Mistake 6: Forgetting getComputedStyle When Reading Variables

Mistake 7: Media Query Override Not in :root

Project Overview

Stage 1 · Setup: Define Your Design Tokens

Lesson 65 complete! 🎉

Global Variables · The `:root` Selector

The `var()` Function · Using Your Variables

Mistake 2: Forgetting `var()` When Using the Variable

Mistake 7: Media Query Override Not in `:root`