Overrode TogetherJS default css to make the extension look more like Review Board.

This change adds some CSS to make the TogetherJS UI fit in better with Review
Board's color scheme and design.

Testing Done:
Launch 'Chat' by clicking the Chat tab located on the right side of the header
bar. The following views on any page illustrate the new changes:

- The right TogetherJS menu should be dark and similar to the header bar.
- The pop-up titled "Invite a friend" should mimic containers in the admin
  panel.
- Clicking on the first icon in the TogetherJS menu, the secondary menu that
  pops up mimics the pull down shown the Support tab in the header bar is
  hovered over.
- Clicking the last icon (Chat) or third icon (Audio Chat) in the TogetherJS
  menu, the pop-up should also mimic the containers found in the admin panel.

Reviewed at https://reviews.reviewboard.org/r/6066/