Function: Default custom background callback.

Function: Renders the Custom CSS style element.

Function: Fetches the `custom_css` post for a given theme.

Function: Fetches the saved Custom CSS content for rendering.

Function: Updates the `custom_css` post for a given theme.

Function: Adds callback for custom TinyMCE editor stylesheets.

Function: Displays header video URL.

Function: Retrieves header video settings.

Function: Checks whether a custom header is set or not.

Function: Checks whether the custom header video is eligible to show on the current page.