Thanks for the suggestion. It sounds interesting to hear. I will try to add this feature in my free time.
There are already some usercripts and extensions with this basic feature, so you can check them out for inspiration.
Yes of course you can. You just need to change the following two line from the script.
Thank you for quick reply.
This worked fine, and I wanted to try exactly the same thing, but I asked you first because I wanted to make sure I won't break anything.
I think you should make red color default in settings, it should work fine in both light and dark themes.