Missing support for dark theme

[iakov]

Generic issue for dark mode/dark theme issues Related #837

1. With Qt5.15 should work generic approach with QT_QPA_PLATFORMTHEME(-platformtheme windows:dark=2), but some clean-up in TS code is required.

2. Useful workaround for Qt5.12.

3. For Linux DEs QT_QPA_PLATFORMTHEME=qt5ct maybe the way to go.

[iakov]

Proposed solution: move to Qt5.15 before any attempts to fix it.

[iakov]

Upd: Added Qt6 info into description.