CSS custom properties improve the flexibility and maintainability of the theme system through variables. It starts with --, such as --primary-color, and calls through var(), supports global and local definitions, and can be dynamically modified through JavaScript. When implementing, the default theme is usually defined in :root, other theme styles are defined in the class, and topic changes are implemented by switching the class name. It is recommended to use the setTheme function to modify the body class name and combine it with local storage to remember user preferences. Notes include: compatibility issues, coverage of variable scope affecting values, increasing debugging difficulty, and avoiding excessive abstraction. The rational use of CSS variables can effectively simplify topic management, and pay attention to details that are easily overlooked.

When implementing a theme system, using CSS custom properties (which are commonly referred to as CSS variables) is a very practical and flexible way to do it. Not only do they make style management clearer, they also dynamically switch topics without reloading pages.

What are CSS custom properties?

CSS custom properties allow you to declare variables in CSS and reuse them throughout the stylesheet. These variables start with two hyphens, such as --primary-color: #333 and are called via var(--primary-color) .

Their greatest advantage lies in inheritance and operability . You can define variables in :root as global variables, or nest them in a certain class or component as local variables. What's more, JavaScript can easily modify them, which is very theme-to-switch.

How to organize the topic variable structure?

To use CSS variables for multiple topics, it is recommended to design a variable naming structure first. The usual practice is:

• Define the default theme in :root
• Define other topic variables under specific class names, such as .theme-dark

 :root {
  --color-bg: #ffff;
  --color-text: #000;
  --primary-color: blue;
}

.theme-dark {
  --color-bg: #111;
  --color-text: #eee;
  --primary-color: cyan;
}

In this way, you only need to add/remove the .theme-dark class to the body or container, and you can switch themes with one click.

How to operate the topic switching

The core of implementing topic switching is JavaScript to dynamically modify the class name of the DOM element or directly set variables. The recommended approach is to control the class name instead of setting variables one by one, which makes it easier to maintain.

For example:

 function setTheme(themeName) {
  document.body.className = themeName;
}

If there are more topics in the future, you just need to extend the CSS file without changing the JS. If you want to be a little more flexible, you can also combine data attributes or local storage to remember user preferences.

Frequently Asked Questions and Precautions

There are some easy things to ignore when using CSS variables to do topics:

• Browser compatibility : Modern browsers basically support it, but if you need to be compatible with IE, you have to find an alternative.
• Variable scope : The scope of a variable is determined by the CSS selector it is located in, and the nesting level affects the final value.
• Debugging difficulty : Sometimes variables are overwritten and are not easy to find, so you can use the browser developer tools to view the current calculated value.
• Don't overuse : Although variables are powerful, not all colors or sizes need to be abstracted into variables, keeping them simpler to maintain.

Basically that's it. Using CSS custom properties well can make your theme system flexible and easy to maintain, and the details that are not complicated but easily overlooked are also worth paying attention to.

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