docs: 📖 update customize-theme

docs/react/customize-theme.en-US.md

@@ -97,6 +97,7 @@ Follow [Use in create-react-app](/docs/react/use-with-create-react-app).
 Another approach to customize theme is creating a `less` file within variables to override `antd.less`.
 
 ```css
+@import '~antd/es/style/themes/default.less';
 @import '~antd/dist/antd.less'; // Import Ant Design styles by less entry
 @import 'your-theme-file.less'; // variables to override above
 ```

docs/react/customize-theme.zh-CN.md

@@ -97,6 +97,7 @@ module.exports = {
 另外一种方式是建立一个单独的 `less` 变量文件，引入这个文件覆盖 `antd.less` 里的变量。
 
 ```css
+@import '~antd/es/style/themes/default.less';
 @import '~antd/dist/antd.less'; // 引入官方提供的 less 样式入口文件
 @import 'your-theme-file.less'; // 用于覆盖上面定义的变量
 ```