Revisions UI: Redesigned slider handlers.

Also:
* Fix diff header styling in grey admin theme
* Prevent an overlap of the compare two checkbox on smaller screens

props melchoyce, DrewAPicture, adamsilverstein. fixes #24276.

git-svn-id: https://core.svn.wordpress.org/trunk@24219 1a063a9b-81f0-0310-95a4-ce76da25c4cd