Restyled notification messages.

This change makes the notification message pop-ups fit in better with Review
Board's visual look.

Testing Done:
1. Launch Review Together in one browsers.
2. Click "Add" button and open the provided URL in another browser. (Note the
   popup in this "Add window")

3. In the second browser, the notification window upon load should look
   similar to the "Add window".
4. Exit the second browser.
5. A pop up in the first browser should have a notification message saying the
   peer has quit. This window will resemble the body of the "Add window".

Reviewed at https://reviews.reviewboard.org/r/6214/