From a QCAD user:
How can I change the background color from black to white?
The background color can be adjusted under:
Edit > Application Preferences > Graphics View > Colors > Graphics View > Background
Optionally, the color adjustment feature can be turned on, to make sure that white lines on a white background are displayed as black and vice versa:
Edit > Application Preferences > Graphics Views > Appearance > Prevent white on white / black on black display