I switched to the WP 2014 theme and the same issue happens. With Subscriptions deactivated, everything looks OK; with Subscriptions activated, the CSS is broken (or missing).
I have switched it back to OP2 for now.
Should we try turning off all the WP plugins to see if it’s somehow a conflict with one of those?
Or uninstalling and reinstalling Subscriptions?
Or manually putting in a call for the CSS to be loaded for the Forum page?