Customizer: Adapt the colors to the selected admin scheme colors.

Uses the $highlight-color variable to override the hardcoded colors used in the customizer UI.

Props dlh, joen.
Fixes #50547.

Built from https://develop.svn.wordpress.org/trunk@48371


git-svn-id: https://core.svn.wordpress.org/trunk@48140 1a063a9b-81f0-0310-95a4-ce76da25c4cd