R1:2a5dae4fcb91

Minor Topic CSS tweaks

Summary: Cleans up the topic UI a little more, I think this feels nice for some reason.

Test Plan: visit a room with and without a topic, desktop, tablet, mobile

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17711