Trying to get to saved themes in Firefox Color doesn't work on some machines depending on the screen size / display. The tab next to "Custom Backgrounds" is overlaid by the huge display of the application menu. One one of my computers you can see it just below the enlarged icon, but on my laptop you can't get to it at all.

I can see it on my desktop PC - barely. On my laptop the icon overlays it completely and I can't get to saved themes. I have tried playing with zoom, but that didn't help. The default / recommended scale in Windows for two different laptops I have is 150% with resolution 1290 x 1200. If you change that to 100%, THEN you can see the saved themes, but I doubt most people would figure that out. Even at 125% I can't see it. I am attaching an example from my laptop where I changed the SCALE set to 125% (again, 150% is recommended by Windows).