Don't hardcode colors

[Frozenlock]

For example: https://github.com/Day8/re-com/blob/master/src/re_com/modal_panel.cljs#L49 and https://github.com/Day8/re-com/blob/master/src/re_com/splits.cljs#L120

The components aren't compatible with bootstrap dark themes because of this.
IMO colors shouldn't be hardcoded. A possible middle ground could be to use classes.
(Re-com already needs its own CSS anyway.)