[native] Fix appearance of AppsDirectory in light mode

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

Use colors from colors.js instead of hardcoded hex values. Don't think the color choice for modalBackground in light mode is great... but that can be addressed later. This is more about having the UI handle both light/dark mode "reasonably."

Test Plan:
Before:

After:

Things look as expected.

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

Reviewed By: ginsu, rohan, O2 Blocking Reviewers, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5612