docs: display all less variables (#7604)

src/count-down/README.md

@@ -163,3 +163,13 @@ Use [ref](https://vuejs.org/v2/api/#ref) to get CountDown instance and call inst
 | start | Start count down | -         | -            |
 | pause | Pause count down | -         | -            |
 | reset | Reset count down | -         | -            |
+
+### Less Variables
+
+How to use: [Custom Theme](#/en-US/theme).
+
+| Name                    | Default Value     | Description |
+| ----------------------- | ----------------- | ----------- |
+| @count-down-text-color  | `@text-color`     | -           |
+| @count-down-font-size   | `@font-size-md`   | -           |
+| @count-down-line-height | `@line-height-md` | -           |

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

@@ -176,7 +176,17 @@ export default {
 | --- | --- | --- | --- |
 | start | 开始倒计时 | - | - |
 | pause | 暂停倒计时 | - | - |
-| reset | 重设倒计时，若`auto-start`为`true`，重设后会自动开始倒计时 | - | - |
+| reset | 重设倒计时，若 `auto-start` 为 `true`，重设后会自动开始倒计时 | - | - |
+
+### 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)。
+
+| 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` | -    |
 
 ## 常见问题