How do I set meta tags for my forum? Do they have to be set exclusively for the theme you're using?
Yes you have to set them for each theme.
It will be in templates -> ungrouped templates -> headerinclude.
Meta tags are pretty redundant these days.
Google SEO provides dynamic meta descriptions for threads. Then you just need to add meta descriptions to other pages, like the index.
If you add to many meta tags Google will punish you. Just don't keyword stuff and only add legit keywords but I doubt it will make a difference.
Google doesn't bother with keywords at all, completely ignores them.
I could care less about the keywords and if they even affect the forum at all. It's the meta description that's important.