Customizer: Set more colors to inherit from admin theme.

Remove color overrides in customizer CSS that are already set in admin theme colors. Set colors to admin theme colors where currently hard coded.

Props madhavishah01, noruzzaman, shailu25, joedolson.
Fixes #64731.

git-svn-id: https://develop.svn.wordpress.org/trunk@61860 602fd350-edb4-49c9-b593-d223f7449a82