The coupling is in the right direction: designing possibly very advanced CSS to get a desired appearance from a given good markup, instead of compromising markup to simplify CSS.
What kind of fragile markup and problematic CSS global rules are you worrying about?
I’m wondering that too. If I’m writing a SPA, styles are usually scopped to the component with a base styles for UI atomic elements. And some conventions around spacing. If it’s a website, it will be separated in layouts and small components Bootstrap-like. An update will be identified as a variant or a special case.
As for coding this, search all files is usually a godsend when refactoring. You can use ripgrep if the editor does not have a good implementation.
You want to preserve the theme an consistency of the UI, but adapt its functionality. For this type of changes the semantic version is problematic.
You don't like global styles? Don't use global styles, period.
Even without adopting Nue and staying on React, CRA and Next comes out with CSS Modules out of the box. Angular and Vue have their own equivalent.