Changing the background color in options does nothing. It always stays black.
You may solve your problem with below settings:
Go to Option > Appearance > Graphic Colors > Background > #FFFFFF (from drop-down) > Click OK (and restart LibreCAD).
Because the color you select remains the same even after changing color from Select Color Window, but won't change due to drop-down selection did not change (see the color code when you change accordingly).