I’m afraid that was based on my faith in the customisability of Discourse, rather than my ability to achieve it. 
I think to add buttons you’d need to override the template, which needs more than a bit of CSS. There’s quite a bit of info on creating theme components and overriding templates in the Developer’s guide to Discourse Themes.
Though if you want to add a ‘Join this Community!’ button then it’s worth mentioning that I think the campaign banner only shows for logged in users. You may be better off exploring something like Versatile Banner or Welcome link banner as an alternative for the anonymous users.