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

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

props bramd.
fixes #26603 for trunk.
Built from https://develop.svn.wordpress.org/trunk@27119


git-svn-id: https://core.svn.wordpress.org/trunk@26986 1a063a9b-81f0-0310-95a4-ce76da25c4cd