hide `background-image` in admin menu for existing dashicons

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@748191 b8457f37-d9ea-0310-8a92-e5e31aec5664
editor top border color now matches editor bottom border color

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@748192 b8457f37-d9ea-0310-8a92-e5e31aec5664
tinymce: remove background color of `html` element (fixes issue with Twenty Thirteen theme) 

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@748194 b8457f37-d9ea-0310-8a92-e5e31aec5664