fix(theme-classic): validate options properly (#7755)

* fix(theme-classic): validate options properly

* improve normalization

* fix doc

website/docs/api/themes/theme-classic.md

@@ -18,3 +18,44 @@ npm install --save @docusaurus/theme-classic
 If you have installed `@docusaurus/preset-classic`, you don't need to install it as a dependency.
 
 :::
+
+## Configuration {#configuration}
+
+Accepted fields:
+
+```mdx-code-block
+<APITable>
+```
+
+| Option | Type | Default | Description |
+| --- | --- | --- | --- |
+| `customCss` | <code>string[] \| string</code> | `[]` | Stylesheets to be imported globally as [client modules](../../advanced/client.md#client-modules). Relative paths are resolved against the site directory. |
+
+```mdx-code-block
+</APITable>
+```
+
+:::note
+
+Most configuration for the theme is done in `themeConfig`, which can be found in [theme configuration](./theme-configuration.md).
+
+:::
+
+### Example configuration {#ex-config}
+
+You can configure this theme through preset options or plugin options.
+
+:::tip
+
+Most Docusaurus users configure this plugin through the preset options.
+
+:::
+
+```js config-tabs
+// Preset Options: theme
+// Plugin Options: @docusaurus/theme-classic
+
+const config = {
+  customCss: require.resolve('./src/css/custom.css'),
+};
+```