[web] Avoid global styles in accept-invite-modal.css

Summary:
This solves ENG-3945, a push-blocking issue on master.

For more context on the issue, see this Linear comment.

Test Plan:
Confirm that the issue is resolved in the following places:

1. Chat header
2. Add app modal (press "+" button)
3. User settings page
4. Members modal

Reviewers: atul, inka, tomek, ginsu

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D7998