color schemes `_minimal.scss`: don't override variables, if they are already defined; added collapse button variables

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@761728 b8457f37-d9ea-0310-8a92-e5e31aec5664
color schemes: prevent blue flash of current menu item background, when toggling menu state

git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@761731 b8457f37-d9ea-0310-8a92-e5e31aec5664