Quick and dirty fix to titlebar properties to make our gui presets work again.
Ok, there's your quick fix for the gui preset titlebars (see the discussion in !538 (merged)). I simply git-diffed all tweaks to default.css since 2.13.0, extracted the titlebar rules from that, and concatenated the resulting css to the other css files (excluding default.css and dejavu.css, of course). Job done. Still needs to be tested, hence WIP for now.