* Custom CSS is associated with a given theme and is displayed in an inline `style` element at the `wp_head` hook after the `wp_print_styles` is called so that it overrides any enqueued stylesheets.
* A `wp_get_custom_css()` function is used for accessing the CSS associated with the current theme (or another theme) and a `wp_get_custom_css` filter for manipulating it.
* CSS is managed in customizer via a new "Additional CSS" section with a single `textarea` control.
* `WP_Customize_Section::$description_hidden` is introduced for hiding extended descriptions in customizer sections behind a help toggle as done with panels.
* CSS is stored in a `custom_css` post type with the theme (stylesheet) slug as the `post_name`.
* `WP_Customize_Custom_CSS_Setting` is introduced to handle validation of CSS, previewing, and persisting the CSS to the `custom_css` post type.
* The `custom_css` setting is tied to a new `unfiltered_css` capability which maps to `unfiltered_html` by default.
* Escaping the message in the notification template is removed to allow markup (`code` tags) to be rendered.
See https://make.wordpress.org/core/2016/10/11/feature-proposal-better-theme-customizations-via-custom-css-with-live-previews/
Props johnregan3, celloexpressions, folletto, westonruter.
Fixes#35395.
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