macOS: Use dark gradients for title and status bar in dark mode

These were hardcoded to light colors, which made the
QMainWindow status bar look out of place and made the
(light) text hard to read.

Hardcode to dark colors for DarkAqua which more or less
match the native look. Keep the optimization where the
Gradients are stored in static variabless.

Change-Id: I3e75b42c41d3e2d18e4bc0f17d950a702ccad662
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Gabriel de Dietrich (DO NOT ADD TO REVIEWS) <gabriel.dedietrich@gmail.com>