Update the UI styles for Playlist Settings.

Props melchoyce for the design.
See #26631.


Built from https://develop.svn.wordpress.org/trunk@27487


git-svn-id: http://core.svn.wordpress.org/trunk@27331 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add a filter and document it: `playlist_styles`. Allows users to roll their own playlist themes.

Props DrewAPicture for the docs.
See #26631.


Built from https://develop.svn.wordpress.org/trunk@27488


git-svn-id: http://core.svn.wordpress.org/trunk@27332 1a063a9b-81f0-0310-95a4-ce76da25c4cd