docs(theme): add config info to docs

Author: carwack

website/pages/theme.mdx

@@ -252,4 +252,26 @@ export default {
     tooltip: 1800,
   },
 };
+```
+
+## Config
+
+The theme's config is to provide global settings that are used by different
+parts of the Chakra UI system.
+
+| Property                  | Description                                                                                                 | Default  |
+| ------------------------- | ----------------------------------------------------------------------------------------------------------- | -------- |
+| `cssVarPrefix`            | The prefix to use for the generated CSS custom properties                                                   | `chakra` |
+| `initialColorMode`        | The initial color mode your application should start with. <br reset/> Can be either `light` or `dark` mode | `light`  |
+| `useSystemColorMode`      | If `true`, the chakra system will update color mode <br reset/> based on your system preferences            | `false`  |
+
+You can leverage the `extendTheme` function to override a specific theme config
+property.
+
+```js
+export default {
+  config: {
+    cssVarPrefix: "ck",
+  },
+};
 ```