Support Operating System's color as the Background Color

[maverick74]

To better integrate with the Operating System's colors, i think it would be nice if we could set the "Background Color" to use the Operating System's theme color, as Firefox's default theme does.

Fox example, i'm in KDE Plasma, i would like to maintain the background color equal to the system's theme and change all the other colors.

Now i have to set a specific color to the background... :(