What CSS tools do big companies use with React?

Hi there,

I'm still trying to figure out what kind of CSS framework/setup/tools to use in my current workflow (mainly with React SPAs), and I'm starting to get genuinely curious about the choices made by big companies.

Old plain CSS? SASS/LESS? JS inline styles? PostCSS + CSS modules? Styled components?

What do Facebook, Netflix, Yahoo, Atlassian & co. use?