Full Screen Window Border Color Option

Many of us now use large monitors but sometimes we still need to run a client connection in full screen mode.  On my ultra-wide screens, this often creates letterbox bands on left and right of the remote session window.  This color is currently a muddy black or grey and I would like to choose the color of this background filler.  Personally, I want pure black to lower the amount of light projected from the monitor as I work this way. (sample image below)