[native] Fix appearance of ThreadAncestors in light mode

Summary:
Context: https://linear.app/comm/issue/ENG-2052/re-introduce-light-mode-in-native

Basically use a color from colors.js for the rightArrow instead of hardcoding "white".

Test Plan:
Before:

After:

Things look as expected.

Reviewers: tomek, marcin, ginsu, rohan, abosh, varun, O2 Blocking Reviewers, ashoat

Reviewed By: ginsu, O2 Blocking Reviewers, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5613