Add dark app theme

[Korb]

Currently, you can choose from the following themes in Settings:

• Default (need to restart)
• windowsvista
• windowsvista
• Windows
• Fusion

(that’s right, partly with a lowercase letter, partly with a capital letter) In addition, another list is available below, “User theme”, consisting of two options:

• default
• valknut

As I understand from the names, none of them are night UI color themes. I suggest adding a black theme to the set out of the box so that you can work with the application in a darkened room without setting the monitor to the minimum brightness. I also find the “Follow OS color theme” checkbox useful.