CSS 變數的驚人細節 - 使用 var() 和很酷的範例

This is the second half of my CSS Variable post, the first half is here.
In this article we'll look into the details of var(). And two cool examples:

• Animation using CSS Variables
• Pure CSS dark mode toggle with system setting detection

Using var()

The var() accesses custom property values (CSS variables). Its syntax is as follows:

var( <custom-property-name>, <fallback-value>? )
</fallback-value></custom-property-name>

Basic Rules

1. The first parameter must be a CSS variable: Direct values, such as var(20px), will result in an error, as var() only accepts custom property names.

2. var() cannot replace property names: In other words, you cannot write something like var(--prop-name): 20px; because var() is limited to use in property values only.
   

.foo {
  margin: var(20px); /* Error, 20px is not a CSS variable */

  --prop-name: margin-top;
  var(--prop-name): 20px; /* Error, cannot use var() this way */
}

Detailed Behaviors

1. var(--b, fallback_value) Fallbacks: The second parameter acts as a fallback value, used when --b is invalid.

2. var(--c,) Syntax with an Empty Fallback: If the fallback value is left empty, the syntax remains valid and will default to an empty value if --c is invalid.

3. Multiple Comma: In var(--d, var(--e), var(--f), var(--g)), everything after the first comma is treated as fallback, so if --d is invalid, the var() expression evaluates var(--e), var(--f), var(--g) as one fallback, to determine the result.

4. var() as a Complete CSS Token: The function acts as a complete CSS token, like 20px would. Therefore, var(--size)var(--unit) will not create 20px and is considered invalid.

5. Using initial with CSS Variables: Assigning initial to a CSS variable means it is invalid. To display initial as a value, it must be placed in the fallback.

6. url() and var() Usage: Since url() is treated as a complete CSS token, you need to define the full url() within the variable.
   

:root {
  /* 1. */
  margin: var(--b, 20px); /* Uses 20px if --b is invalid */

  /* 2. */
  padding: var(--c,) 20px; /* Falls back to 20px if --c is invalid */

  /* 3. */
  font-family: var(--fonts, "lucida grande", tahoma, Arial); /* Uses fallback font stack if --fonts is invalid */

  /* 4. */
  --text-size: 12;
  --text-unit: px;
  font-size: var(--text-size)var(--text-unit); /* Invalid, as it does not resolve to 12px */

  /* 5. */
  --initialized: initial;
  background: var(--initialized, initial); /* Results in background: initial */

  /* 6. */
  --invalid-url: "https://useme.medium.com";
  background: url(var(--invalid-url)); /* Invalid, as url() cannot parse var() */

  --valid-url: url(https://useme.medium.com);
  background: var(--valid-url); /* Correct usage */
}

Variable Resolution and Scope

CSS variables, like other CSS properties, follow CSS-specific rules for scope and specificity. Understanding how these factors affect CSS variables allows for more precise control.

Global and Scoped Variables:
Variables defined in :root are applied globally, while those defined in selectors have a more limited scope.

   :root {
     --main-color: blue; /* Globally applied */
   }

   .container {
     --main-color: green; /* Scoped, applies only within .container */
   }

Priority by Specificity:
Higher specificity will override lower specificity for CSS variables.

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

   .section {
     --main-color: green; /* Overrides :root definition */
   }

   .section p {
     color: var(--main-color); /* Shows green */
   }

   p {
     color: var(--main-color); /* Shows blue */
   }

   <div>



<p><strong>Calculating Values Based on Specificity Order:</strong> <br>
Like CSS properties, variables are resolved based on specificity in ascending order.<br>
</p>

<pre class="brush:php;toolbar:false">   :root {
     --red: 255;
     --green: 255;
     --blue: 255;
     --background: rgb(var(--red), var(--green), var(--blue));
   }

   .box {
     --green: 0;
     background: var(--background); 
   }

In this example, the background color of .box remains white, as --background was resolved to rgb(255, 255, 255) before .box redefined --green: 0.

Reevaluating Variables with Pseudo-Classes:
Variables change based on pseudo-class states when defined at the same level.

   :root {
     --red: 255;
     --green: 255;
     --blue: 255;
   }

   .box {
     --background: rgb(var(--red), var(--green), var(--blue));
     background: var(--background);
   }

   .box:hover {
     --green: 0; /* Changes background color on hover */
   }

---

Next, let’s explore some advanced use cases for CSS variables:

Usage Example A: Animations

CSS variables cannot be directly animated because the browser cannot infer the data type. To resolve this, use @property to define the variable's type and initial value, enabling the browser to understand how to animate the variable.

@property --green {
  syntax: "<number>";
  initial-value: 255;
  inherits: false;
}

.section {
  padding: 5em;
  background: rgb(50, var(--green), 50);
  transition: --green 0.5s;
}

.section:hover {
    --green: 50;
}
</number>

<div>



<p>In this example, @property is used to declare --green as a <number> type with an initial value of 255. When hovering over .section, --green changes to 50, creating a smooth color transition effect.</number></p>

<p>CodePen example</p>


<hr>

<h2>
  
  
  Usage Example B: Light/Dark Mode Toggle
</h2>

<p>If you want to implement theme switching for light and dark modes, it’s helpful to extract color values into variables that adjust automatically based on the prefers-color-scheme setting. Here’s how you can manage this using CSS variables.<br>
</p>

<pre class="brush:php;toolbar:false">:root {
  --background-color: #FBFBFB;
  --container-background-color: #EBEBEB;
  --headline-color: #0077EE;
  --text-color: #333333;
}

@media (prefers-color-scheme: dark) {
  :root {    
    --background-color: #121212;
    --container-background-color: #555555;
    --headline-color: #94B2E6;
    --text-color: #e0e0e0;
  }
}

Adding a Manual Toggle that Aligns with System Preferences

While the system setting controls the theme by default, we may want to give users the option to manually toggle between light and dark themes. To achieve this, we can add a checkbox to toggle the state. Ideally, when the checkbox is selected, it indicates dark mode, and when unselected, it represents light mode.

However, CSS cannot automatically detect system settings and change the checkbox state accordingly, especially in dark mode. To handle this limitation, we can use CSS variables and the :has() selector to control theme switching based on the checkbox state.

I wanted to try achieving this entirely with CSS, but since a user’s system may be set to either light or dark mode, CSS alone can’t automatically check the checkbox in dark mode.

If we can’t move the mountain, we’ll route the path. Here’s the workaround:

• We’ll use CSS to create a toggle switch, where the visual “OFF” state represents light mode, and “ON” represents dark mode.

• When system sets to light mode: When the checkbox is unselected, it corresponds to the “OFF” state (light mode). When selected, it corresponds to the “ON” state (dark mode).

• When system sets to dark mode: Since the system preference is reversed, the visual state also inverts. When the checkbox is unselected, it corresponds to “ON” (dark mode). When selected, it corresponds to “OFF” (light mode).

To achieve this effect, we need two main elements:

First: Variables that Change Based on System Setting and Checkbox State

:root {
  --background-color: #FBFBFB;
  --container-background-color: #EBEBEB;
  --headline-color: #0077EE;
  --text-color: #333333;
}

:root:has(input[type="checkbox"]:checked) {
  --background-color: #121212;
  --container-background-color: #555555;
  --headline-color: #94B2E6;
  --text-color: #e0e0e0;
}

@media (prefers-color-scheme: dark) {
  :root {    
    --background-color: #121212;
    --container-background-color: #555555;
    --headline-color: #94B2E6;
    --text-color: #e0e0e0;
  }

  :root:has(input[type="checkbox"]:checked) {
    --background-color: #FBFBFB;
    --container-background-color: #EBEBEB;
    --headline-color: #0077EE;
    --text-color: #333333;
  }
}

Second: Toggle Behavior Based on System Settings for checked State and ON/OFF Representation

The light and dark mode CSS properties are reversed depending on the system setting.

/* Toggle Switch Styles */
.switch {
  position: relative;
  display: inline-block;
  width: 60px;
  height: 34px;
}

/* Hide the checkbox element to style a custom switch */
.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

/* Slider styling for the switch background */
.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: var(--slider-bg, #ccc);
  transition: 0.4s;
  border-radius: 34px;
}

/* Slider knob styling */
.slider:before {
  position: absolute;
  content: "";
  height: 26px;
  width: 26px;
  left: 4px;
  bottom: 4px;
  background-color: white;
  transition: 0.4s;
  border-radius: 50%;
}

/* Dark mode styles: make the switch look "checked" by default */
@media (prefers-color-scheme: dark) {
  .slider {
    background-color: #94b2e6;
  }
  .slider:before {
    transform: translateX(26px); /* Move knob to the right */
  }

  /* Invert checked state in dark mode to look "unchecked" */
  input:checked + .slider {
    background-color: #ccc;
  }
  input:checked + .slider:before {
    transform: translateX(0); /* Move knob to the left */
  }
}

/* Light mode styles: make the switch look "unchecked" by default */
@media (prefers-color-scheme: light) {
  .slider {
    background-color: #ccc;
  }
  .slider:before {
    transform: translateX(0); /* Knob on the left */
  }

  /* Invert checked state in light mode to look "checked" */
  input:checked + .slider {
    background-color: #94b2e6;
  }
  input:checked + .slider:before {
    transform: translateX(26px); /* Move knob to the right */
  }
}

Simplifying Variable Setup with CSS Variable Tricks

Here we’ll use Space Toggle technique to simplify variable settings. Here’s the code, followed by an explanation of how it works:

:root {
  --ON: initial; /* Default state variable to use for switching colors */
  --OFF: ; /* Alternative state variable for switching colors */

  /* Set default color variables based on light mode */
  --light: var(--ON);
  --dark: var(--OFF);

  /* Define custom properties for colors used in light and dark modes */
  --background-color: var(--light, #fbfbfb) var(--dark, #121212);
  --container-background-color: var(--light, #ebebeb) var(--dark, #555555);
  --headline-color: var(--light, #0077ee) var(--dark, #94b2e6);
  --text-color: var(--light, #333333) var(--dark, #e0e0e0);
}

:root:has(input[type="checkbox"]:checked) {
  --light: var(--OFF);
  --dark: var(--ON);
}

The key here is in the line --background-color: var(--light, #fbfbfb) var(--dark, #121212);. Here, the background color depends on the values of --light and --dark, effectively simulating an if/else in the property.

How does it work? Initially, --light: var(--ON); and --ON: initial; make --ON an invalid state. Meanwhile, --OFF is set as an empty string. When applied to var(--light, #fbfbfb) var(--dark, #121212), the invalid --light variable will default to #fbfbfb, and the valid --dark variable (empty) allows --background-color to equal #fbfbfb.

All the other color variables follow the same logic, adjusting based on the state of --light and --dark. This way, each color variable only needs to be defined once.

Switching states becomes simple. If dark mode is active, use --light: var(--OFF); and --dark: var(--ON);. In light mode, reverse them. Though not immediately intuitive, this method is currently the most effective with CSS. If there are better solutions, they are worth exploring.

Complete example: CodePen Example

---

Summary

CSS continues to evolve, with CSS variables available in major browsers since 2016. New features like @property and :has() are expanding CSS variables’ flexibility even further. Combined with other new tools, CSS variables are becoming more powerful—for instance, they can now enhance scroll-driven animations to create visually dynamic effects. As a core element for storing state in CSS, much like variables in any programming language, a solid understanding of CSS variables will prove invaluable for more sophisticated styling and design down the road.

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References

• https://stackoverflow.com/questions/42330075/is-there-a-way-to-interpolate-css-variables-with-url/42331003#42331003
• https://kizu.dev/cyclic-toggles/#was-this-always-possible
• https://dev.to/afif/what-no-one-told-you-about-css-variables-553o
• https://hackernoon.com/cool-css-variable-tricks-to-try-uyu35e9
• https://lea.verou.me/blog/2020/10/the-var-space-hack-to-toggle-multiple-values-with-one-custom-property/