This includes:
* Removing the deprecated `screen_icon()` function call.
* Replacing the deprecated `get_current_theme()` function call with `get_option( 'current_theme' )`.
* Using `wp_get_theme()->display( 'Name' )` explicitly instead of relying on `WP_Theme`'s `__toString()` method, for clarity.
Follow-up to [6334], [20039], [20040], [20042], [20508], [26537], [41274].
Props Presskopp, cu121, viralsampat, costdev, tomjdv, sabernhardt, SergeyBiryukov.
Fixes#54833.
Built from https://develop.svn.wordpress.org/trunk@53626
git-svn-id: http://core.svn.wordpress.org/trunk@53185 1a063a9b-81f0-0310-95a4-ce76da25c4cd
3e622b1d75