What do you want to do exactly?
A. You can run a multisite installation with BP network activated. There each subsite can use it’s own theme, only the mainsite needs a theme that is comfortable with BP. But since BP 1.7 there should every theme be supported.
B. If you wish to keep the styles of Tersus theme on your main site, but have to override some functionality – You could make an own child theme of Tersus theme and override it’s behaviour.
To oversimplify it: We want the main site to look like tersus, and the community to not look like tersus.
Huddle or Buddy or any number of BP themes have a much cleaner, more user friendly, organized and rich design than the part of tersus that enables BP.
It there a way to have just the community part have a different theme – I know there’s only one theme per installation, but is there a workaround?
Thinking about your suggestion of overriding BP behavior on a child theme – I am just not developer enough to build a theme and construct it differently. I could have the templates for child theme and another BP theme side by side and I still wouldn’t know what to scavenge!
Did I get you right: ‘Community’ means the BP features like groups, members, acivity – ‘main site’ means other WP pages and your WP blog? So, no that’s not possible. You’d have to either enhance your theme or separate the site into community on the main site of a mulsitite install and the rest on a subsite.
Yes! That’s exactly it, and I was afraid it wasn’t possible. Looks like I would have to alter the theme’s buddypress components significantly, and I don’t have the skills for that. 🙁
Regarding the other solution, Can I have buddypress as the main and the installation in the root be a subsite? Do I just not display anthing that is not to do with community?
With multisite I mean both sites inside of one installation: https://codex.wordpress.org/Create_A_Network
There you can activate BP on the main site, then all the community stuff will go there. You can add another site (subdomain) and run your WP stuff there. But that’s not that easy, too. Read throught the linked page if this would be a possibility for you.
Otherwise maybe the easiest would be just to switch completely to the new theme…