Oh. So the real problem is that your camera can't use a dark cloth or dark cloth equivalent. Oh. Brighter screen won't help much.
I focus my 2x3 Graphics with a 3.6x Toyo loupe, also use a Cambo SF 320 in-line viewing hood, use any of a T-20 reflex viewer, T-21 in-line viewer or C-288 binocular reflex viewer on the 4x5. Focusing with all of the Cambo hoods is a little iffy, not enough magnification. Takes several passes through best focus to get it right. The C-288 is the best of the lot but is bulky so difficult to transport and heavy. It pulls the focusing panel away from the back, has to be held in place. Or else.