Add support for "macos_dark_titlebar_color"

[isaksamsten]

So I'm back with a new pull request.

This introduces a new configuration variable called macos_dark_titlebar_color which sets the titlebar color to a color distinct from the one set for macos_titlebar_color if custom or dark color mode is toggled. It is also changed upon system theme change.

I'm not sure if it's the best solution but I want to be able to have a titlebar that matches my pdf background color and this makes it happen.

What do you think?

Best, Isak