docs: display all less variables (#7604)

src/image/README.md

@@ -104,3 +104,17 @@ Vue.use(Lazyload);
 | default `v2.9.0` | Custom the content below the image |
 | loading          | Custom loading placeholder         |
 | error            | Custom error placeholder           |
+
+### Less Variables
+
+How to use: [Custom Theme](#/en-US/theme).
+
+| Name                                | Default Value       | Description |
+| ----------------------------------- | ------------------- | ----------- |
+| @image-placeholder-text-color       | `@gray-6`           | -           |
+| @image-placeholder-font-size        | `@font-size-md`     | -           |
+| @image-placeholder-background-color | `@background-color` | -           |
+| @image-loading-icon-size            | `32px`              | -           |
+| @image-loading-icon-color           | `@gray-4`           | -           |
+| @image-error-icon-size              | `32px`              | -           |
+| @image-error-icon-color             | `@gray-4`           | -           |

src/image/README.zh-CN.md

@@ -137,6 +137,20 @@ Vue.use(Lazyload);
 | loading          | 自定义加载中的提示内容     |
 | error            | 自定义加载失败时的提示内容 |
 
+### 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 |
+| ----------------------------------- | ------------------- | ---- |
+| @image-placeholder-text-color       | `@gray-6`           | -    |
+| @image-placeholder-font-size        | `@font-size-md`     | -    |
+| @image-placeholder-background-color | `@background-color` | -    |
+| @image-loading-icon-size            | `32px`              | -    |
+| @image-loading-icon-color           | `@gray-4`           | -    |
+| @image-error-icon-size              | `32px`              | -    |
+| @image-error-icon-color             | `@gray-4`           | -    |
+
 ## 常见问题
 
 ### 如何引用本地图片？