Use .screen-reader-text class instead of an alt attribute for overlay button labels on Themes screen.

Merges [27119] to the 3.8 branch.

props bramd.
fixes #26603.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@27875 602fd350-edb4-49c9-b593-d223f7449a82