Qimage has various skins that allow you to change the look of the UI but you are saying the same settings on one machine produces a gray UI and on another, the same background of Qimage windows and dialogs is black? The default skin has a gray background for all windows and dialogs so if they look black on one particular system, you are probably going to need to correct the problem with that system: calibrate and profile the monitor so gray no longer looks black.
Mike is suggesting that you try this: See screen snap 038.
These are Qimage UI color selections.
As Mike said, if you mean the whole monitor, a lot more information is needed.
Fred