Standard SCSS processed via Webpack with simplified BEM naming convention (namespaces class wrapper) is the way I’m going.
Component SCSS is handled via SCSS @import, this also means I can use SCSS @mixins and @placeholders.
You can colocate component CSS files or keep them in a css/components folder. Just change the @import path.
Only import the main.scss in client app.js so Webpack can work it’s magic.
This is standard Sass/SCSS CSS approach and is framework agnostic.
See this article for details: Styling React components in SASS