For this, you’ll need the knowledge of CSS programming language (which isn’t that hard, really. But for safety measure, you need to be familiar with HTML too)
However, changing the BG color during the game is only controllable by the reader/player, as @hustlertwo pointed out.
From my knowledge of CScript, I think that’s not possible but I haven’t really used it too much so feel free to correct me
As for the case which @Hakai_no_Megami mentioned, I think such a thing requires some sort of third party plugins, which wouldn’t be the first usage of those in CoG/HG games. Heroes Rise apparently used quite a few back in the days when CScript was much less efficient and capable than it is now
Thanks for that link. I was actually wondering if it was possible to change the colour of the screen to make it clearer of you were switching between character viewpoints and the like, but it looks like it can only be a single change for the whole story.
I must insist that I’ve seen it done in SoS:TMC. I’m no expert, but I can think of a way to do it that might work, but I’d have to actually try to do it to see whether or not it works.