* 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.
Built from https://develop.svn.wordpress.org/trunk@39209
git-svn-id: http://core.svn.wordpress.org/trunk@39149 1a063a9b-81f0-0310-95a4-ce76da25c4cd
* 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.
Built from https://develop.svn.wordpress.org/trunk@38829
git-svn-id: http://core.svn.wordpress.org/trunk@38772 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Weston Ruter