Customizer: Add admin color scheme class to body element.

This changeset injects a CSS class called `admin-color-{slug}` into the `body` element in the Customizer screen based on the admin color scheme setting.

Props soyebsalar01, westonruter, wildworks.
Fixes #64415.

git-svn-id: https://develop.svn.wordpress.org/trunk@61385 602fd350-edb4-49c9-b593-d223f7449a82

Author: t-hamano

src/wp-admin/customize.php

@@ -147,6 +147,8 @@
 	$body_class .= ' rtl';
 }
 $body_class .= ' locale-' . sanitize_html_class( strtolower( str_replace( '_', '-', get_user_locale() ) ) );
+$admin_color = get_user_option( 'admin_color' );
+$body_class .= ' admin-color-' . sanitize_html_class( is_string( $admin_color ) ? $admin_color : '', 'fresh' );
 
 if ( wp_use_widgets_block_editor() ) {
 	$body_class .= ' wp-embed-responsive';