Add dark UI theme with QPalette and custom css theme support

[kaygdev]

Add dark Ui theme based on QPalette and provide a simple way to use custom css file. For the dark UI the Qt style is changed to fusion to avoid problems with other styles (WindowsVista style does allow to change button color).

Only tested on Linux (I currently have no Windows system with development environment, sorry).

The chosen colors are only a suggestion, they can be replaced.

[trollixx]

Hey @kaygdev, thank you for submitting, I'll review this after the Qt WebEngine port is complete. Very close to merging that pull request.

[CuteWaterBeary]

Any update on this?

[trollixx]

Qt WebEngine offers native dark theme support, so this is no longer required. See #811 for more information.