Ensure active tabs have a background color to avoid any awkward bleed-through in case of overlap. props TobiasBg. fixes #27116.

Built from https://develop.svn.wordpress.org/trunk@27749


git-svn-id: http://core.svn.wordpress.org/trunk@27586 1a063a9b-81f0-0310-95a4-ce76da25c4cd