Add focus style

[charakterziffer]

I like that theme very much. Sadly I hempers the use of keyboard navigation. When I tab through my file explorer nautilus or through web forms there’s no hint which button is in focus at the moment. The Adwaita themes give focussed buttons an orange border. Maybe a thick dark grey border would be a possible focus indicator for Flatabulous? (a thicker border because grey is not a colour that stands out alone)