How to enable New Closed Topic Icon?

Continuing the discussion from Discourse 2.5.0.beta3 Release Notes:

This has been released for a while and I am on a beta4, but I do not see a way to enable the new icon on my instance. Do I need to issue some manual icon refresh command from the app container?

We should edit that (removed), the replacement icons (we tried a few) weren’t as clear based on various feedback so we went back to the lock.