R1:8d398f6f4a69

Fix variable name for theme_class

Summary: Fixes T12622. This variable is mis-named.

Test Plan: Visit a room I have not joined.

Reviewers: epriestley, amckinley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12622

Differential Revision: https://secure.phabricator.com/D17767