seems like an expo thing, since expo is really in control of that layer, isn’t it?

Hi. What approach does fulcro users use to write css?

Fulcro-css with co-located definitions just writes a style element straight to the DOM. See the docs.

some users make their own css, etc.

I tend to use semantic UI on my own projects, and customize the general facets according to the docs there. Any component-centric one-offs are nice to co-locate to that specific component, so I use fulcro-css for those.