Dear Zed, I read your reply to cjfeola on 26th April, it seems I have the same problems after the wordpress 5 update. Your answer is about previous WP updates, can I follow the same advice? (I was running the latest Nirvana theme already and deactivated and activated it again, but no change) Your answer was:
Since version 1.2 Nirvana uses the companion Serious Settings plugin to activate its settings page (the reasons behind this are detailed in the 1.2 update announcement).
As you’re still using a (very) old version of the theme on the site you will need to update the theme past v1.2 for the settings plugin to work (as described here).