Remove custom colors and background

Remove all the properties for custom colors: there is already an api to change colors of selected hyerarchies and is Kirigami.Theme

This api besides being redundant and somewhat buggy, is never used (only user so far seems audiotube) and makes assumptions on what the look of the tabbar is (one already broken in the latest restyle as is talking about an "highlight line" which doesn't exists anymore since a couple of years)