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Declaring variables in CSS - Mini Project: Build & Apply

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Declaring CSS Variables for Theming
📖 Scenario: You are creating a simple webpage theme using CSS variables. CSS variables help you keep colors and fonts consistent and easy to change.
🎯 Goal: Declare CSS variables for primary color, secondary color, and base font size. Use these variables in the CSS to style a heading and a paragraph.
📋 What You'll Learn
Declare CSS variables inside the :root selector
Create variables named --primary-color, --secondary-color, and --base-font-size
Use the variables to style the h1 color, p color, and font size
Ensure the CSS is valid and applies correctly to the HTML
💡 Why This Matters
🌍 Real World
CSS variables are used in real websites to create themes and make design changes easy and consistent.
💼 Career
Knowing how to declare and use CSS variables is important for front-end developers to build maintainable and scalable stylesheets.
Progress0 / 4 steps
1
Create the CSS root selector
Write a CSS rule for the :root selector to start declaring variables.
CSS
Hint

The :root selector targets the whole document for global CSS variables.

2
Declare CSS variables inside :root
Inside the :root selector, declare these CSS variables exactly: --primary-color with value #3498db, --secondary-color with value #2ecc71, and --base-font-size with value 1.2rem.
CSS
Hint

CSS variables start with two dashes -- and are declared with a colon and value.

3
Use CSS variables to style heading and paragraph
Write CSS rules for h1 and p elements. Set h1 color to var(--primary-color), p color to var(--secondary-color), and p font size to var(--base-font-size).
CSS
Hint

Use var(--variable-name) to access CSS variables inside other CSS rules.

4
Complete the CSS with a body background color
Add a CSS rule for the body element that sets its background color to var(--secondary-color).
CSS
Hint

Use the body selector to set the page background color using the CSS variable.

Practice

(1/5)
1. What is the main purpose of declaring CSS variables inside :root?
easy
A. To limit variables only to the :root selector
B. To create variables that only work in inline styles
C. To make variables available globally throughout the CSS
D. To declare variables that only apply to JavaScript

Solution

  1. Step 1: Understand the role of :root in CSS

    The :root selector targets the highest-level element in the document, usually the <html> element.

  2. Step 2: Recognize variable scope

    Declaring variables inside :root makes them global, so they can be used anywhere in the CSS.

  3. Final Answer:

    To make variables available globally throughout the CSS -> Option C

  4. Quick Check:

    Global variables = :root declaration [OK]
Hint: Declare variables in :root for global access [OK]
Common Mistakes:
  • Thinking variables declared in :root are local
  • Confusing CSS variables with JavaScript variables
  • Assuming variables only work inline
2. Which of the following is the correct syntax to declare a CSS variable named --main-color with the value #3498db inside :root?
easy
A. :root { --main-color: #3498db; }
B. :root { main-color = #3498db; }
C. :root { $main-color: #3498db; }
D. :root { var(--main-color): #3498db; }

Solution

  1. Step 1: Recall CSS variable declaration syntax

    CSS variables start with two dashes -- and are declared with a colon : inside a selector block.

  2. Step 2: Check each option

    :root { --main-color: #3498db; } uses correct syntax: --main-color: #3498db;. Others use invalid symbols or keywords.

  3. Final Answer:

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

  4. Quick Check:

    Correct syntax = --name: value; [OK]
Hint: Use --name: value; inside :root [OK]
Common Mistakes:
  • Using = instead of : to assign values
  • Missing the double dash -- prefix
  • Trying to use var() in declaration
3. Given the CSS below, what color will the paragraph text be?
:root {
  --text-color: #ff0000;
}
p {
  color: var(--text-color);
}
medium
A. Red
B. Black
C. Blue
D. Green

Solution

  1. Step 1: Identify the variable value

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

  2. Step 2: Check how the variable is used

    The paragraph uses color: var(--text-color);, so it will use the red color.

  3. Final Answer:

    Red -> Option A

  4. Quick Check:

    Variable value applied = red [OK]
Hint: Match variable value with var(--name) usage [OK]
Common Mistakes:
  • Confusing hex codes with color names
  • Forgetting to use var() to apply variables
  • Assuming default color if variable is declared
4. What is wrong with this CSS code?
:root {
  --bg-color #ffffff;
}
body {
  background-color: var(--bg-color);
}
medium
A. Background color property is misspelled
B. Using var() incorrectly to apply variable
C. Variable name missing double dashes '--' prefix
D. Missing colon ':' after variable name in declaration

Solution

  1. Step 1: Check variable declaration syntax

    The declaration --bg-color #ffffff; is missing a colon : between the variable name and value.

  2. Step 2: Verify usage of variable

    The usage background-color: var(--bg-color); is correct, so the error is only in declaration.

  3. Final Answer:

    Missing colon ':' after variable name in declaration -> Option D

  4. Quick Check:

    Declaration syntax requires ':' [OK]
Hint: Always put ':' between variable name and value [OK]
Common Mistakes:
  • Forgetting colon ':' in variable declaration
  • Using var() without parentheses
  • Misspelling property names
5. You want to create a theme with two colors: --primary-color and --secondary-color. You want --secondary-color to be 50% transparent version of --primary-color. Which CSS variable declaration correctly achieves this?
hard
A. :root { --primary-color: 0, 0, 255; --secondary-color: rgba(var(--primary-color), 0.5); }
B. :root { --primary-color: #0000ff; --secondary-color: #0000ff80; }
C. :root { --primary-color: #0000ff; --secondary-color: rgba(var(--primary-color), 0.5); }
D. :root { --primary-color: #0000ff; --secondary-color: var(--primary-color, 0.5); }

Solution

  1. Step 1: Understand CSS variable limitations

    CSS variables hold values as strings; you cannot directly use var(--primary-color) inside rgba() expecting it to split into RGB components.

  2. Step 2: Check each option

    :root { --primary-color: #0000ff; --secondary-color: rgba(var(--primary-color), 0.5); } tries to use rgba(var(--primary-color), 0.5) but --primary-color is a hex string, so this won't work.
    :root { --primary-color: 0, 0, 255; --secondary-color: rgba(var(--primary-color), 0.5); } declares --primary-color as RGB components but CSS variables cannot hold multiple values like that easily.
    :root { --primary-color: #0000ff; --secondary-color: #0000ff80; } uses a hex color with alpha channel #0000ff80 which is blue with 50% opacity, correctly representing a transparent version.
    :root { --primary-color: #0000ff; --secondary-color: var(--primary-color, 0.5); } misuses var() with a second parameter that is not opacity.

  3. Final Answer:

    :root { --primary-color: #0000ff; --secondary-color: #0000ff80; } -> Option B

  4. Quick Check:

    Use hex with alpha for transparency in variables [OK]
Hint: Use hex with alpha channel for transparent colors [OK]
Common Mistakes:
  • Trying to use var() inside rgba() with hex colors
  • Declaring RGB as comma-separated string in variable
  • Misusing var() fallback parameter as opacity