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.