Agreed +1 it is a glaring visual brightness issue sometimes. The only workaround, make a much larger Page Size, and set the Page Background Black (or Fog Of War ON), and move your Map contents to somewhere in the middle. Thus creating large black margins around the playable space. An added convenience, the margins (if hidden in Fog Of War) can be used to store DM tokens, monsters and such. However it would be better if we can reskin, colorize, or Theme the background and other parts of Roll20 to fit our game style.