Rebass does all styling inline. You can override per instance (pass in `style` prop), or "globally" via the React `context`. Example themes [1] can be seen in the demo [2] ("Config" dropdown in top right).

[1] https://github.com/jxnblk/rebass/tree/master/demo/configurat... [2] http://jxnblk.com/rebass/demo/