CSS custom properties are a key tool for implementing dynamic styles. 1. Custom attributes start with --, such as --main-color, defined in:root and called through var() to improve code maintainability; 2. Dynamic changes in theme colors are achieved through class name switching, combining JavaScript and local storage to support user preferences; 3. CSS variables can be modified in real time through JavaScript, which is suitable for color selectors, A/B tests and other scenarios; 4. Pay attention to avoiding nested use of var(), which is incompatible with IE, and requires reasonable handling of downgrade solutions.
Of course, the following is an article with clear structure and practical content:

Direct response to the title: CSS custom properties are an effective way to implement dynamic styles
CSS custom properties (also called CSS variables) provide developers with a way to define and use reusable values ??in style sheets. They not only improve the maintainability of the code, but also combine JavaScript to achieve more flexible dynamic style control.

1. How to define and use CSS custom properties
Custom properties start with two hyphens, such as --main-color
, which is usually defined in :root
, so that they can be accessed throughout the document.
:root { --main-color: #3498db; --font-size: 16px; }
Called via var()
function in other CSS rules:

button { background-color: var(--main-color); font-size: var(--font-size); }
- Pay attention to naming specifications when defining, and it is recommended to use semantic names.
- It is recommended to manage variables uniformly, for example, in a separate
_variables.css
file.
2. Tips for dynamically changing theme colors
Using custom attributes and class name switching, multi-theme support can be easily achieved. For example, switch between dark mode and light mode.
:root { --bg-color: #ffffff; --text-color: #333333; } .dark-mode { --bg-color: #1a1a1a; --text-color: #f0f0f0; }
In HTML, you only need to add or remove the dark-mode
class to <body>
and it will take effect.
JavaScript operations are also very simple:
document.body.classList.toggle('dark-mode');
- It is recommended to save the theme status in local storage, so that users can still retain preferences after refreshing the page.
- System preferences can be automatically detected in combination with media query.
3. Real-time style updates with JavaScript
Because CSS variables are dynamic, their values ??can be modified at runtime through JS to achieve real-time style changes.
document.documentElement.style.setProperty('--main-color', '#e74c3c');
This feature is very suitable for:
- User-defined theme settings (such as color selector)
- Dynamically adjust UI element styles according to user behavior
- Different variants in A/B tests are displayed
A simple example is to have the user enter a color value and apply it immediately:
<input type="color" id="colorPicker">
const input = document.getElementById('colorPicker'); input.addEventListener('input', (event) => { document.documentElement.style.setProperty('--main-color', event.target.value); });
4. Precautions and compatibility considerations
Although modern browsers generally support custom properties, there are still some things to note:
- Do not use
var()
in nested, for example,var(--color: var(--fallback))
is illegal. - Custom properties cannot be used as part of the property selector.
- Older versions of browsers (such as IE) do not support them at all and require downgrade processing or Polyfill.
If the project must be compatible with the old environment, it is recommended to use it only on non-critical paths, or work with preprocessor variables.
Basically that's it. Although CSS custom attributes are small, if used properly, it can significantly improve development efficiency and user experience.
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