First off, this setting is session dependant, meaning it will NOT take effect until you re-start UG.
Now, you need to know FOR SURE which customer defaults file you're using. If you're working in English units, then it's ug_english.def. If you're working in metric units, then it's ug_metric.def. If you're working in a custom environment, such as the GM PDL Toolkit, then you need to find the .def file that the custom environment is using.
When you have found the correct customer defaults file, search for:
UG_shaded_graduated_background_color
or
UG_wireframe_graduated_background_color
Each individual line is an RGB value (red, green, blue). Set all of your values that are not commented out to 255 & that will result in your background always being white, regardless of whether in shaded or wireframe display mode. Don't worry about the background being set to graduated, as there is NOT a setting (that I am aware of) for controlling the background if you set it to plain.
Hope this helps.
Tim Flater
Senior Designer
Enkei America, Inc.