[native] Fix appearance of "Join Chat" in light mode

Summary: We actually don't want the color of the text here to change based on theme. Rather it should be determined by the threadColor behind it.

Test Plan:
Things look as expected:

Before:

After:

Reviewers: tomek, marcin, ginsu, rohan, ashoat

Reviewed By: ashoat

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5622