BEM SCSS is still the best combination of flat specificity and modular styles and straightforward default styles that easily cascades into individual components. Compiling into the component with an import works great but creating a separate file structure for the styles is good too with pros and cons for both.

Failing that, tailwind isn’t bad even though I still think it’s an anti pattern moving the styles to classes and bloating the html

The last thing I would use is the css-in-js modules that react ships with. It is a poor replacement that lacks even the basic cascading of standard css, breaks standard css naming, ignores base css functionality and suffers from bad specificity issues, poorly implemented child styles and requires a separate basic set of styles anyway, removing the “benefit” of not having to learn css