arc-flatabulous is a fork of the famous Arc theme. The author tweaked the colors in a few places and changed the button colors so it looks more like MacOSX (huge doubt). Same as installing Arc-Theme, you need to delete the old stuff first, if you’ve installed it before. (Skip this if you haven’t.) sudo rm -rf /usr/share/themes/{Arc-Flatabulous,Arc-Flatabulous-Darker,Arc-Flatabulous-Dark} rm -rf ~/.local/share/themes/{Arc-Flatabulous,Arc-Flatabulous-Darker,Arc-Flatabulous-Dark} rm -rf ~/.themes/{Arc-Flatabulous,Arc-Flatabulous-Darker,Arc-Flatabulous-Dark}