message: docs: improve dark theme guide
diff: @@ -28,6 +28,15 @@ In takes effect globally, making all Vant components on the page dark.
  <van-config-provider theme="dark">...</van-config-provider>
  ```
  +The theme prop will not change the text-color or background-color of the page, you can set it manually like this:
  +
  +```css
  +.van-theme-dark body {
  + text-color: #f5f5f5;
  + background-color: black;
  +}
  +```
  +
  > Tips: The theme prop will not change the background color of the page, you need to set it manually.
  ### Switch Theme
  @@ -212,6 +221,8 @@ CSS variables in Vant are divided into **basic variables** and **component varia
  - The basic variables can only be modified through the `:root` selector.
  - The component variables can be modified through the `:root` selector and `ConfigProvider` component.
  +You can also use the `.van-theme-light` and `.van-theme-dark` class selector to modify basic or component variables in light or dark mode individually.
  +
  #### Variables List
  There are all **Basic Variables** below, for component CSS Variables, please refer to the documentation of each component.
  