Using variables in CSS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a CSS variable?

A CSS variable is a custom property that stores a value you can reuse throughout your CSS. It starts with two dashes, like --main-color.

beginner

How do you declare a CSS variable?

You declare a CSS variable inside a selector using the syntax: --variable-name: value;. For example, :root { --main-color: #3498db; } sets a variable named --main-color.

beginner

How do you use a CSS variable in your styles?

Use the var() function with the variable name. For example, color: var(--main-color); applies the value stored in --main-color.

beginner

Why use CSS variables instead of repeating values?

CSS variables make it easy to change a value in one place and have it update everywhere. This saves time and keeps your styles consistent.

beginner

Where is the best place to declare global CSS variables?

Declare global CSS variables inside the :root selector. This makes them available everywhere in your CSS.

How do you declare a CSS variable named --bg-color with the value #fff?

A background-color: --bg-color #fff;

B :root { --bg-color: #fff; }

C var(--bg-color) = #fff;

D body { bg-color = #fff; }

CSS variables are declared inside selectors with two dashes and a colon, like --bg-color: #fff;.

How do you use a CSS variable named --text-color to set text color?

A color: var(--text-color);

B color: --text-color;

C var(text-color);

D text-color: var(--text-color);

Use the var() function with the variable name to apply its value.

What is the advantage of using CSS variables?

A They are required for all CSS.

B They make CSS run faster.

C They replace HTML tags.

D They let you reuse values and update them easily.

CSS variables help keep your styles consistent and easy to update.

Where should you declare CSS variables to make them available everywhere?

A :root selector

B Inside each HTML tag

C Only inside <style> tags

D Inside JavaScript files

Declaring variables in :root makes them global in your CSS.

Which of these is a correct CSS variable name?

A main-font

B $main-font

C --main-font

D var-main-font

CSS variable names must start with two dashes, like --main-font.

Explain how to create and use a CSS variable to set a background color.

Think about how you store a color once and reuse it.

Describe the benefits of using CSS variables in a website's style.

Imagine changing one color everywhere by changing just one place.

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