docs: update component CSS variable names (#8857)

src/count-down/README.md

@@ -181,12 +181,12 @@ Use [ref](https://v3.vuejs.org/guide/component-template-refs.html) to get CountD
 | pause | Pause count down | -         | -            |
 | reset | Reset count down | -         | -            |
 
-### Less Variables
+### CSS Variables
 
-How to use: [Custom Theme](#/en-US/theme).
+The component provides the following CSS variables, which can be used to customize styles. Please refer to [ConfigProvider component](#/en-US/config-provider).
 
-| Name                    | Default Value     | Description |
-| ----------------------- | ----------------- | ----------- |
-| @count-down-text-color  | `@text-color`     | -           |
-| @count-down-font-size   | `@font-size-md`   | -           |
-| @count-down-line-height | `@line-height-md` | -           |
+| Name                         | Default Value     | Description |
+| ---------------------------- | ----------------- | ----------- |
+| --van-count-down-text-color  | `@text-color`     | -           |
+| --van-count-down-font-size   | `@font-size-md`   | -           |
+| --van-count-down-line-height | `@line-height-md` | -           |

src/count-down/README.zh-CN.md

@@ -193,13 +193,13 @@ export default {
 
 ### 样式变量
 
-组件提供了下列 Less 变量，可用于自定义样式，使用方法请参考[主题定制](#/zh-CN/theme)。
+组件提供了下列 CSS 变量，可用于自定义样式，使用方法请参考 [ConfigProvider 组件](#/zh-CN/config-provider)。
 
-| 名称                    | 默认值            | 描述 |
-| ----------------------- | ----------------- | ---- |
-| @count-down-text-color  | `@text-color`     | -    |
-| @count-down-font-size   | `@font-size-md`   | -    |
-| @count-down-line-height | `@line-height-md` | -    |
+| 名称                         | 默认值            | 描述 |
+| ---------------------------- | ----------------- | ---- |
+| --van-count-down-text-color  | `@text-color`     | -    |
+| --van-count-down-font-size   | `@font-size-md`   | -    |
+| --van-count-down-line-height | `@line-height-md` | -    |
 
 ## 常见问题