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.

Built from https://develop.svn.wordpress.org/branches/3.8@27875


git-svn-id: http://core.svn.wordpress.org/branches/3.8@27706 1a063a9b-81f0-0310-95a4-ce76da25c4cd