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.

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git-svn-id: http://core.svn.wordpress.org/branches/3.8@26890 1a063a9b-81f0-0310-95a4-ce76da25c4cd