gui: resurrect old dark theme as black theme
gui: resurrect old dark theme as black theme

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3676

imsodin/syncthing,jryans/syncthing,le9i0nx/syncthing,lkwg82/syncthing,xduugu/syncthing,kluppy/syncthing,xduugu/syncthing,uok/syncthing,syncthing/syncthing,ProactiveServices/syncthing,Kudalufi/syncthing,Kudalufi/syncthing,kamadak/syncthing,Kudalufi/syncthing,xduugu/syncthing,plouj/syncthing,kamadak/syncthing,kamadak/syncthing,uok/syncthing,ProactiveServices/syncthing,uok/syncthing,imsodin/syncthing,cherylquirion/syncthing,AudriusButkevicius/syncthing,kluppy/syncthing,AudriusButkevicius/syncthing,imsodin/syncthing,brgmnn/syncthing,rumpelsepp/syncthing,xduugu/syncthing,brgmnn/syncthing,rumpelsepp/syncthing,brgmnn/syncthing,lkwg82/syncthing,le9i0nx/syncthing,imsodin/syncthing,cherylquirion/syncthing,le9i0nx/syncthing,brgmnn/syncthing,uok/syncthing,jryans/syncthing,wweich/syncthing,ProactiveServices/syncthing,cherylquirion/syncthing,jryans/syncthing,imsodin/syncthing,syncthing/syncthing,cherylquirion/syncthing,AudriusButkevicius/syncthing,nrm21/syncthing,plouj/syncthing,Kudalufi/syncthing,nrm21/syncthing,kamadak/syncthing,lkwg82/syncthing,syncthing/syncthing,le9i0nx/syncthing,kluppy/syncthing,plouj/syncthing,syncthing/syncthing,nrm21/syncthing,wweich/syncthing,rumpelsepp/syncthing,rumpelsepp/syncthing,kluppy/syncthing,ProactiveServices/syncthing,wweich/syncthing,plouj/syncthing,nrm21/syncthing,wweich/syncthing,lkwg82/syncthing,syncthing/syncthing,syncthing/syncthing,AudriusButkevicius/syncthing,jryans/syncthing