* Add `customize_value_custom_css` filter to `WP_Customize_Custom_CSS::value()` method.
* Introduce `customize_update_custom_css_post_content_args` filter in `WP_Customize_Custom_CSS::update()` method.
* Make clear that `wp_get_custom_css()` and `wp_get_custom_css` filter are specifically for obtaining the value to render/display. Eliminate use of `wp_get_custom_css()` when getting the setting value. Use the underlying `post_value` directly when `is_previewed`.
* Move anonymous functions handing JS previewing for `custom_logo`, `custom_css`, and `background` into named functions on the `wp.customize.settingPreviewHandlers` to allow plugins to override/extend preview logic.
* Update `_custom_background_cb` to always print a `style` tag wen in the customizer preview, and update background preview logic to replace existing style element instead of appending a new style to the head so that background changes don't unexpectedly override any Custom CSS in the preview's stylesheet cascade.
Props westonruter, georgestephanis.
See #22058.
Fixes#38672.
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