Using variables in CSS - Mini Project: Build & Apply

Using CSS Variables for Theming

📖 Scenario: You are creating a simple webpage that uses colors for background and text. To make it easy to change these colors later, you want to use CSS variables.

🎯 Goal: Build a CSS stylesheet that defines color variables and applies them to the page background and text color.

📋 What You'll Learn

Define CSS variables for background color and text color

Use the CSS variables in the body selector

Ensure the colors are easy to update by changing only the variables

💡 Why This Matters

🌍 Real World

CSS variables are used in real websites to create themes and make styling easier to update without changing many lines of code.

💼 Career

Knowing CSS variables is important for front-end developers to write clean, maintainable, and scalable stylesheets.

1

Create CSS variables for colors

Create CSS variables called --bg-color and --text-color inside the :root selector. Set --bg-color to #f0f0f0 and --text-color to #333333.

CSS

:root {
  /* Define CSS variables here */
  /* Your code here */
}

Hint

Use the :root selector to define global CSS variables with --variable-name: value;.

2

Add body selector to use variables

Add a body selector and set its background-color to the CSS variable --bg-color and color to the CSS variable --text-color using the var() function.

CSS

:root {
  --bg-color: #f0f0f0;
  --text-color: #333333;
}

body {
  /* Use CSS variables for colors here */
  /* Your code here */
}

Hint

Use var(--variable-name) to access CSS variables inside property values.

3

Add a heading style using variables

Add a h1 selector and set its color to the CSS variable --text-color.

CSS

:root {
  --bg-color: #f0f0f0;
  --text-color: #333333;
}

body {
  background-color: var(--bg-color);
  color: var(--text-color);
}

h1 {
  /* Set heading color using variable */
  /* Your code here */
}

Hint

Use the same var(--text-color) to keep heading color consistent.

4

Update variables for a dark theme

Change the CSS variables inside :root to create a dark theme by setting --bg-color to #222222 and --text-color to #eeeeee.

CSS

:root {
  --bg-color: #f0f0f0;
  --text-color: #333333;
}

body {
  background-color: var(--bg-color);
  color: var(--text-color);
}

h1 {
  color: var(--text-color);
}

/* Update variables for dark theme here */
/* Your code here */

Hint

Change the variable values inside :root to update the theme colors easily.

Practice

1. What is the main purpose of CSS variables?

easy

A. To store reusable values like colors and sizes

B. To create new HTML elements

C. To write JavaScript code inside CSS

D. To add comments in CSS files

Solution

1. Step 1: Understand what CSS variables do

   CSS variables hold values that can be reused throughout the stylesheet, such as colors or font sizes.

2. Step 2: Compare options with this purpose

   Only To store reusable values like colors and sizes describes storing reusable values; others describe unrelated tasks.

3. Final Answer:

   To store reusable values like colors and sizes -> Option A

4. Quick Check:

   CSS variables = reusable values [OK]

Hint: CSS variables store values you reuse often [OK]

Common Mistakes:

• Thinking CSS variables create HTML elements
• Confusing CSS variables with JavaScript
• Believing CSS variables add comments

2. Which is the correct way to define a CSS variable for a primary color globally?

easy

A. :root { --primary-color: #3498db; }

B. body { primary-color: #3498db; }

C. :root { primary-color = #3498db; }

D. html { --primary-color #3498db; }

Solution

1. Step 1: Recall CSS variable syntax

   Variables are defined with two dashes and a colon inside a selector, usually :root for global scope.

2. Step 2: Check each option's syntax

   :root { --primary-color: #3498db; } uses correct syntax: :root { --name: value; }. Others have missing dashes, wrong selectors, or wrong assignment.

3. Final Answer:

   :root { --primary-color: #3498db; } -> Option A

4. Quick Check:

   Define variables with --name: value; inside :root [OK]

Hint: Use :root and --var-name: value; to define variables [OK]

Common Mistakes:

• Missing double dashes before variable name
• Using = instead of : for assignment
• Defining variables outside :root for global use

3. Given the CSS below, what color will the paragraph text be?

:root { --main-color: #ff0000; } p { color: var(--main-color); }

medium

A. Blue

B. Green

C. Black

D. Red

Solution

1. Step 1: Identify the variable value

   The variable --main-color is set to #ff0000, which is red.

2. Step 2: Check how the variable is used

   The paragraph's color is set using var(--main-color), so it uses the red color.

3. Final Answer:

   Red -> Option D

4. Quick Check:

   var(--main-color) = #ff0000 (red) [OK]

Hint: Match variable value hex to color name [OK]

Common Mistakes:

• Ignoring the variable and using default color
• Confusing hex codes with other colors
• Not using var() to apply variable

4. What is wrong with this CSS code?

:root { --font-size 16px; } h1 { font-size: var(--font-size); }

medium

A. Variable name should not start with dashes

B. Missing colon after --font-size in variable definition

C. Using var() incorrectly in h1 font-size

D. font-size property cannot use variables

Solution

1. Step 1: Check variable definition syntax

   The variable definition is missing a colon after --font-size; it should be --font-size: 16px;

2. Step 2: Verify usage of var()

   The usage var(--font-size) is correct in h1 font-size property.

3. Final Answer:

   Missing colon after --font-size in variable definition -> Option B

4. Quick Check:

   Variable definitions need colon after name [OK]

Hint: Variable definitions need colon after name [OK]

Common Mistakes:

• Forgetting colon in variable definition
• Thinking var() usage is wrong
• Believing variable names can't start with --

5. You want to create a theme with two colors using CSS variables: primary as blue (#0000ff) and secondary as gray (#888888). How do you apply these variables to style a button's background and border color?

hard

A. :root { --primary: blue; --secondary: gray; } button { background-color: --primary; border: 2px solid --secondary; }

B. :root { primary: #0000ff; secondary: #888888; } button { background-color: primary; border: 2px solid secondary; }

C. :root { --primary: #0000ff; --secondary: #888888; } button { background-color: var(--primary); border: 2px solid var(--secondary); }

D. button { --primary: #0000ff; --secondary: #888888; background-color: var(primary); border: 2px solid var(secondary); }

Solution

1. Step 1: Define variables globally with correct syntax

   Variables must be defined inside :root with double dashes and colon, e.g., --primary: #0000ff;

2. Step 2: Use variables with var() in button styles

   Apply variables using var(--primary) for background and var(--secondary) for border color.

3. Final Answer:

   :root { --primary: #0000ff; --secondary: #888888; } button { background-color: var(--primary); border: 2px solid var(--secondary); } -> Option C

4. Quick Check:

   Define with --name: value; use with var(--name) [OK]

Hint: Define variables in :root and use var(--name) to apply [OK]

Common Mistakes:

• Omitting -- in variable names
• Not using var() when applying variables
• Defining variables inside button instead of :root