Updates to Dashicons classes.

props melchoyce.
fixes #26227.


git-svn-id: https://develop.svn.wordpress.org/trunk@26752 602fd350-edb4-49c9-b593-d223f7449a82
Themes: Avoid overlap with themes with small screenshots and no description.

props matveb.
fixes #26465.


git-svn-id: https://develop.svn.wordpress.org/trunk@26753 602fd350-edb4-49c9-b593-d223f7449a82