Remove button-secondary class from Collapse link in theme installer; unify Collapse button styles in theme installer and Customize screen; add a border around the Collapse icon so it remains visible on dark backgrounds; move the color styles to colors-fresh. Fixes #26230.

git-svn-id: https://develop.svn.wordpress.org/trunk@26389 602fd350-edb4-49c9-b593-d223f7449a82