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

props joedolson for initial patch.
fixes #26602 for trunk.

Built from https://develop.svn.wordpress.org/trunk@27012


git-svn-id: https://core.svn.wordpress.org/trunk@26889 1a063a9b-81f0-0310-95a4-ce76da25c4cd