grayscale scrollbar option for the stamp selection background color

altho i dont want to be blasted with white when i open the stamps menu, some stamps, especially ones that are just linework (including all of the new battlemaps classics) are very hard to see even for someone with 20/20 vision.

I suggest adding a small scroll bar from white to black on top of the page next to the UI size scroll bar so the background color can dynamically change from black to grays to white.