[native] Fix issue with ColorSelector header color

Summary:
Context: https://linear.app/comm/issue/ENG-2354/header-in-color-select-modal-barely-visible-on-dark-mode

Here's where I caused the regression: https://phab.comm.dev/D5619#171440

Basically I set color to modalForegroundLabel in a StyleSheet instead of in an unboundStyles object that gets "bound" with the useStyles(...) hook.

Thanks @kamil for catching this issue.

Test Plan:
Here's how it looks in light mode + dark mode:

Reviewers: kamil, tomek, marcin, ginsu, rohan, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, kamil

Differential Revision: https://phab.comm.dev/D5751