docs: display all less variables (#7604)

src/grid/README.md

@@ -132,3 +132,16 @@ Vue.use(GridItem);
 | default | Custom content |
 | icon    | Custom icon    |
 | text    | Custom text    |
+
+### Less Variables
+
+How to use: [Custom Theme](#/en-US/theme).
+
+| Name | Default Value | Description |
+| --- | --- | --- |
+| @grid-item-content-padding | `@padding-md @padding-xs` | - |
+| @grid-item-content-background-color | `@white` | - |
+| @grid-item-content-active-color | `@active-color` | - |
+| @grid-item-icon-size | `28px` | - |
+| @grid-item-text-color | `@gray-7` | - |
+| @grid-item-text-font-size | `@font-size-sm` | - |