'Gradient Background' color setting has no effect on the Preview in File->Open, i.e. background color of the preview gets its defaults from the syscol.scl file or system_background_color option in config.pro, BUT it is not possible to set the background as blended Gradient.
In Wildfire versions of Pro/E (like Wildfire 5.0) the Gradient Background option controlled both - The main Graphic Window + the Preview Window,
but in CREO 3.0 the Gradient Background option controls only the Graphic Window.
Most of Pro/E users that I know prefer drafting drawings with a black background.
Setting this Black background option, undesirably affects the background of the PREVIEW window - to be BLACK!
It is most needed to have the Gradient Background color setting to control also the PREVIEW Window!