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The only somewhat workable solution for that is to write an extension that does something on every site to extract some color information from it. You would obviously have to decide what it even means for site "to have some specific color" which could be non-trivial. You can perhaps check Vivaldifox github page for pointers on how they do such color extraction.
FYI, I hacked together a proof of concept extension + style that does what you describe. It lives in this repository - the signed extension .xpi can be found in releases and the CSS to be applied via userChrome.css is in the css directory.