[web] Avoid global styles in accept-invite-modal.css
This solves ENG-3945, a push-blocking issue on master.
For more context on the issue, see this Linear comment.
Confirm that the issue is resolved in the following places:
- Chat header
- Add app modal (press "+" button)
- User settings page
- Members modal
Reviewers: atul, inka, tomek, ginsu
Reviewed By: atul
Differential Revision: https://phab.comm.dev/D7998