Dark mode

[irgolic]

As outlined in biolab/orange3#2749

Dark mode is implemented in Orange, but it doesn't look nice with the category colors.
Some work would also need to be put into changing colors of objects such as shadows.

@JakaKokosar, you had dark mode enabled automatically on a new MacOS recently, right? Did we disable that?

[borondics]

@irgolic asked me to post the screenshot from biolab/orange3#5410 here to show the search field text color and bg color.

[borondics]

Would it be possible to add an option to disable the automatic dark/light mode that is currently inherited from the OS? Widgets are not ready for this yet and sometimes things are not functional. An example is below, where there is actually a black curve on balck-ish background (we could correct this in the widget itself but I am not sure we can handle this in a nice way already or not):

[ales-erjavec]

You can go to Preferences -> Style and change Style to Fusion and Color theme to Breeze Light, then restart orange. Unfortunately there is no easy way to use native macos style with different color scheme.