Show a "No themes found" message if the search for an installed theme doesn't produce any results.

props gauravmittal1995 for initial patch.
fixes #28587.
Built from https://develop.svn.wordpress.org/trunk@28890


git-svn-id: http://core.svn.wordpress.org/trunk@28689 1a063a9b-81f0-0310-95a4-ce76da25c4cd