After Stylish's data stealing scandal, using a closed source browser extensiosn has become too risky, unless it's from a widely known softwaare developer... which it's not the case for many of those extensions... so for things I can automate myself, like making changes to CSS as in your example, I went all the way with open-source javascript injection engines, and among all of them in 2023 it's hard to not recommend the superb Violent-Monkey. It features the nicest and most complete UI compared to the alternatives I've tried, and it's got a very well written documentation.
As other have said here, these injection engines are easily capable of altering the CSS... For example following is the code for a script which fixes the awful Instagram white splash-screen which flashes on each page reload:
// ==UserScript==
// @name fix-instagram-white-splashscreen
// @namespace Violentmonkey Scripts
// @match https://www.instagram.com/*
// @grant GM_addStyle
// @version 1.0
// @author AUTHOR-NAME
// @description 8/13/2023, 12:35:17 PM
// ==/UserScript==
GM_addStyle (
`
#splash-screen {
background-color: #333 !important;
}
`
);
That's it. Note that the header part is mostly autopopulated with Violent-Monkey and it can also be modified by the UI without needing to remember all the flags. It's really a powerful tool and I happy I am learning to use it because it can come handy in many different use cases... and for many of those there may be scripts written by other users alrady, as as for Grease Monkey and the others, but I recommend to only use them if you can read their code yourself, because you never know...