You can do this yourself in many browsers. In Pale Moon I just went to Preferences,
Appearance, Color, and changed the background color to red - it could have been any color - and in the menu below selected to show this always. I left text as black. You can forcibly select different font sizes and types too.
Now the forum is bright red with black text, I think I might change that back myself though (especially as the emojis in the pop up box are invisible now until I hover over them when they show up at the bottom, and they show in the post preview, so I guess this is not a completely perfect solution )