CSS custom properties (variables) in Svelte - Step-by-Step Execution

Concept Flow - CSS custom properties (variables)

Define custom property in :root or selector

↓

Use var() function to access property

↓

Browser applies value to CSS property

↓

If property changes, dependent styles update

↓

Styles render with updated values

CSS custom properties are defined once and reused with var(), allowing easy style updates and consistent theming.

Execution Sample

Svelte

:root {
  --main-color: #3498db;
}

button {
  background-color: var(--main-color);
}

Defines a blue color variable and uses it as the button background color.

Execution Table

Step	CSS Rule Processed	Custom Property Defined	Property Used	Computed Value	Visual Effect
1	:root { --main-color: #3498db; }	--main-color = #3498db	N/A	N/A	Variable stored for use
2	button { background-color: var(--main-color); }	No new variable	background-color	#3498db	Button background becomes blue
3	Change --main-color to #e74c3c	--main-color = #e74c3c	background-color	#e74c3c	Button background updates to red
4	Remove --main-color definition	--main-color undefined	background-color	falls back to initial (transparent)	Button background loses color
5	Use var(--main-color, #2ecc71) with fallback	No change	background-color	#2ecc71	Button background becomes green fallback
6	End of style processing	Final state	Final computed styles	Final values applied	Styles rendered on page

💡 CSS processing ends after applying all rules and resolving variables with fallbacks if needed.

Variable Tracker

Variable	Initial	After Step 1	After Step 3	After Step 4	After Step 5	Final
--main-color	undefined	#3498db	#e74c3c	undefined	undefined	undefined

Key Moments - 3 Insights

Why does the button background change color when --main-color is updated?

Because the button uses var(--main-color), when the variable changes (see step 3), the button's background-color updates automatically (step 3 row in execution_table).

What happens if --main-color is not defined and no fallback is given?

The property using var(--main-color) becomes invalid and falls back to the default CSS value (step 4 shows variable undefined and background-color loses color).

How does the fallback value in var() work?

If the variable is undefined, the fallback value is used instead (step 5 shows background-color uses fallback #2ecc71).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of --main-color after step 3?

A undefined

B #e74c3c

C #3498db

D #2ecc71

💡 Hint

Check the 'Custom Property Defined' column at step 3 in the execution_table.

At which step does the button background lose its color because the variable is undefined?

A Step 2

B Step 5

C Step 4

D Step 1

💡 Hint

Look for when --main-color becomes undefined and background-color changes in the execution_table.

If we add a fallback value in var(--main-color, #2ecc71), what color will the button background be when --main-color is undefined?

A #2ecc71

B #e74c3c

C #3498db

D transparent

💡 Hint

See step 5 in execution_table where fallback value is used.

Concept Snapshot

CSS custom properties are variables defined with --name inside selectors like :root.
Use them with var(--name) to apply consistent values.
If variable changes, all uses update automatically.
Fallback values in var() provide defaults if variable is missing.
Great for theming and easy style updates.

Full Transcript

CSS custom properties, also called CSS variables, let you store values like colors or sizes in one place. You define them inside selectors such as :root using --variable-name syntax. Then you use them in your CSS with the var() function. When the variable changes, all places using it update automatically. If a variable is missing, you can provide a fallback value inside var(). This makes styling easier and consistent. For example, defining --main-color in :root and using var(--main-color) for button backgrounds means changing the color in one place updates all buttons. If the variable is removed, the style falls back or becomes default. This visual trace shows how variables are stored, used, updated, and how fallbacks work step-by-step.

Practice

1. What is the main purpose of CSS custom properties (variables) in a Svelte component?

easy

A. To import external CSS files into the component

B. To write JavaScript code inside CSS

C. To store reusable values that can be changed dynamically in CSS

D. To create new HTML elements automatically

Solution

1. Step 1: Understand CSS custom properties

   CSS custom properties allow you to define variables in CSS that store values like colors or sizes.

2. Step 2: Recognize their use in Svelte

   In Svelte, these variables can be used inside style blocks to reuse and dynamically change styles.

3. Final Answer:

   To store reusable values that can be changed dynamically in CSS -> Option C

4. Quick Check:

   CSS variables = reusable style values [OK]

Hint: CSS variables hold reusable style values [OK]

Common Mistakes:

• Thinking CSS variables run JavaScript
• Confusing CSS variables with importing files
• Believing CSS variables create HTML elements

2. Which of the following is the correct syntax to define a CSS custom property named --main-color with the value blue inside a Svelte component's style block?

easy

A. div { --main-color: blue }

B. style { --main-color = blue; }

C. :root { main-color: blue; }

D. :root { --main-color: blue; }

Solution

1. Step 1: Identify correct CSS variable syntax

   CSS variables start with two dashes and use a colon to assign values, e.g., --main-color: blue;.

2. Step 2: Recognize the proper selector for global variables

   The :root selector defines variables globally in CSS.

3. Final Answer:

   :root { --main-color: blue; } -> Option D

4. Quick Check:

   Correct CSS variable syntax uses colon and double dashes [OK]

Hint: Use :root and colon for CSS variables [OK]

Common Mistakes:

• Using equals sign (=) instead of colon (:)
• Omitting the double dashes (--)
• Defining variable without a selector

3. Given the following Svelte component style and markup, what color will the text inside the <p> tag appear?

<style>
  :root {
    --text-color: red;
  }
  p {
    color: var(--text-color);
  }
</style>

<p>Hello World</p>

medium

A. Red

B. Blue

C. Black

D. Green

Solution

1. Step 1: Check the CSS variable definition

   The variable --text-color is set to red inside :root, making it globally available.

2. Step 2: Check the usage in the p selector

   The paragraph text color is set to var(--text-color), which uses the variable value red.

3. Final Answer:

   Red -> Option A

4. Quick Check:

   CSS variable value = red [OK]

Hint: var(--variable) uses the defined CSS variable value [OK]

Common Mistakes:

• Assuming default black color instead of variable
• Confusing variable name or value
• Ignoring :root scope

4. Identify the error in this Svelte component style block that tries to use a CSS custom property:

<style>
  :root {
    --bg-color: #eee
  }
  div {
    background-color: var(--bg-color);
  }
</style>

medium

A. Missing semicolon after #eee in :root block

B. Incorrect variable name syntax, should not use dashes

C. Cannot use var() inside Svelte style blocks

D. Background color property name is wrong

Solution

1. Step 1: Check CSS syntax in :root block

   CSS requires a semicolon after each property value. The line --bg-color: #eee is missing a semicolon.

2. Step 2: Verify other syntax elements

   Variable name syntax with dashes is correct, var() usage is valid, and property name background-color is correct.

3. Final Answer:

   Missing semicolon after #eee in :root block -> Option A

4. Quick Check:

   CSS properties need semicolons [OK]

Hint: Check for missing semicolons in CSS blocks [OK]

Common Mistakes:

• Forgetting semicolons after CSS declarations
• Thinking variable names can't have dashes
• Believing var() is invalid in Svelte styles

5. In a Svelte component, you want to create a button whose background color changes based on a CSS custom property --btn-bg. You want to allow the parent component to set this variable dynamically. Which approach correctly allows this dynamic styling?

<!-- Child.svelte -->
<script>
  export let bgColor = 'lightblue';
</script>

<button style="background-color: var(--btn-bg);">Click Me</button>

<style>
  button {
    color: white;
  }
</style>

How should the parent component pass the background color to the child using CSS custom properties?

hard

A. Pass bgColor prop and use it directly in the child's style attribute without CSS variables

B. Set --btn-bg on the child component's element using inline style: <Child style="--btn-bg: green;" />

C. Define --btn-bg inside the child's <style> block only

D. Use JavaScript to change the button's background color instead of CSS variables

Solution

1. Step 1: Understand CSS variable scope in Svelte

   CSS custom properties can be set on elements and inherited by children. Setting --btn-bg on the child component's root element allows the child's styles to use it.

2. Step 2: Recognize how to pass CSS variables from parent

   The parent can set the CSS variable via inline style on the child component tag, e.g., <Child style="--btn-bg: green;" />, so the child's CSS picks it up.

3. Final Answer:

   Set --btn-bg on the child component's element using inline style: <Child style="--btn-bg: green;" /> -> Option B

4. Quick Check:

   Pass CSS variables via inline style on component [OK]

Hint: Pass CSS variables via inline style on component tag [OK]

Common Mistakes:

• Trying to set CSS variables only inside child styles
• Ignoring CSS variable inheritance and scope
• Using props without connecting to CSS variables