Themes screen: Add aria-describedby attributes to provide better information to screen readers.

Merges [27012] to the 3.8 branch.

props joedolson for initial patch.
fixes #26602.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@27013 602fd350-edb4-49c9-b593-d223f7449a82