Use a brighter blue against dark backgrounds in the admin menu and toolbar.

Not only does this pass contrast guidelines, but it also visually appears closer to the original highlight blue used against light backgrounds.

props melchoyce. fixes #30323.

Built from https://develop.svn.wordpress.org/trunk@30313


git-svn-id: http://core.svn.wordpress.org/trunk@30312 1a063a9b-81f0-0310-95a4-ce76da25c4cd