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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27489 602fd350-edb4-49c9-b593-d223f7449a82
A string was missing its translation wrapper in the Underscore template for video details.

Props Jayjdk.
See #27016.



git-svn-id: https://develop.svn.wordpress.org/trunk@27490 602fd350-edb4-49c9-b593-d223f7449a82