Add a class, `wp-playlist-playing`, to the currently loaded track when tracks are displayed. Add some subtle styles for light and dark playlist themes.

See #27321.


Built from https://develop.svn.wordpress.org/trunk@27489


git-svn-id: http://core.svn.wordpress.org/trunk@27333 1a063a9b-81f0-0310-95a4-ce76da25c4cd
A string was missing its translation wrapper in the Underscore template for video details.

Props Jayjdk.
See #27016.


Built from https://develop.svn.wordpress.org/trunk@27490


git-svn-id: http://core.svn.wordpress.org/trunk@27334 1a063a9b-81f0-0310-95a4-ce76da25c4cd