docs: add advanced theme customization instruction (#519)

* Add docs on how to customize theme

* fix heading level

* extend description

* Add missing classes

* Update exampleSite/content/docs/advanced/customization.md

---------

Co-authored-by: Xin <fuxin1997@gmail.com>

exampleSite/content/docs/advanced/customization.md

@@ -44,6 +44,128 @@ The primary color of the theme can be customized by setting the `--primary-hue`,
 }
 ```
 
+### Further Theme Customization
+
+The theme can be further customized by overriding the default styles via the exposed css classes. An example for customizing the footer element:
+
+```css {filename="assets/css/custom.css"}
+.hextra-footer {
+  /* Styles will be applied to the footer element */
+}
+
+.hextra-footer:is(html[class~="dark"] *) {
+  /* Styles will be applied to the footer element in dark mode */
+}
+
+```
+
+The following classes can be used to customize various parts of the theme.
+
+#### General
+
+- `hextra-scrollbar` - The scrollbar element
+- `content` - Page content container
+
+#### Shortcodes
+
+##### Badge
+
+- `hextra-badge` - The badge element
+
+##### Card
+
+- `hextra-card` - The card element
+- `hextra-card-image` - The card image element
+- `hextra-card-icon` - The card icon element
+- `hextra-card-subtitle` - The card subtitle element
+
+##### Cards
+
+- `hextra-cards` - The cards grid container
+
+##### Jupyter Notebook
+
+- `hextra-jupyter-code-cell` - The Jupyter code cell container
+- `hextra-jupyter-code-cell-outputs-container` - The Jupyter code cell outputs container
+- `hextra-jupyter-code-cell-outputs` - The Jupyter code cell output div element
+
+##### PDF
+
+- `hextra-pdf` - The PDF container element
+
+##### Steps
+
+- `steps` - The steps container
+
+##### Tabs
+
+- `hextra-tabs-panel` - The tabs panel container
+- `hextra-tabs-toggle` - The tabs toggle button
+
+##### Filetree
+
+- `hextra-filetree` - The filetree container
+
+##### Folder
+
+- `hextra-filetree-folder` - The filetree folder container
+
+#### Navbar
+
+- `nav-container` - The navbar container
+- `nav-container-blur` - The navbar container in blur element
+- `hamburger-menu` - The hamburger menu button
+
+#### Footer
+
+- `hextra-footer` - The footer element
+
+#### Search
+
+- `search-wrapper` - The search wrapper container
+- `search-input` - The search input element
+- `search-results` - The search results list container
+
+#### Table of Contents
+
+- `hextra-toc` - The table of contents container
+
+#### Sidebar
+
+- `mobile-menu-overlay` - The overlay element for the mobile menu
+- `sidebar-container` - The sidebar container
+- `sidebar-active-item` - The active item in the sidebar
+
+#### Language Switcher
+
+- `language-switcher` - The language switcher button
+- `language-options` - The language options container
+
+#### Theme Toggle
+
+- `theme-toggle` - The theme toggle button
+
+#### Cody Copy Button
+
+- `hextra-code-copy-btn-container` - The code copy button container
+- `hextra-code-copy-btn` - The code copy button
+
+#### Code Block
+
+- `hextra-code-block` - The code block container
+
+#### Feature Card
+
+- `hextra-feature-card` - The feature card link element
+
+#### Feature Grid
+
+- `hextra-feature-grid` - The feature grid container
+
+#### Breadcrumbs
+
+No specific class is available for breadcrumbs.
+
 ### Syntax Highlighting
 
 List of available syntax highlighting themes are available at [Chroma Styles Gallery](https://xyproto.github.io/splash/docs/all.html). The stylesheet can be generated using the command: