new conversations: fix admin menu styling

fixes CNVS-9295

test plan:
  * open new conversations
  * select a conversation
  * open the cog menu in the header
  * mark the conversation as unread
  * reopen the cog menu
  * verify that the "Mark as Read" entry is correctly styled

Change-Id: Ib363f2dcdc67bd08ee1c29d83ea7901d4cf53d1d
Reviewed-on: https://gerrit.instructure.com/32239
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>