Your theme has a hard CSS code which adds :before {display:table} to all divs. This is a terrible thing. You should consider to contact the theme developer and ask them fix this general hard affection. wpForo style works fine and we can't fix tons of issues made by this theme.

In case the theme developers don't help you, please navigate to Dashboard > Forums > Settings > Styles admin page, put this CSS code in Custom CSS Code textarea. Save it, delete all caches and do Ctrl+F5 on forum front. This code will protect wpForo content from the theme hard CSS code:

#wpforo #wpforo-wrap article:after, #wpforo #wpforo-wrap article:before, #wpforo #wpforo-wrap aside:after, 
#wpforo #wpforo-wrap aside:before, #wpforo #wpforo-wrap details:after, #wpforo #wpforo-wrap details:before, 
#wpforo #wpforo-wrap div:after, #wpforo #wpforo-wrap div:before, #wpforo #wpforo-wrap figcaption:after, 
#wpforo #wpforo-wrap figcaption:before, #wpforo #wpforo-wrap figure:after, #wpforo #wpforo-wrap figure:before, 
#wpforo #wpforo-wrap footer:after, #wpforo #wpforo-wrap footer:before, #wpforo #wpforo-wrap header:after, 
#wpforo #wpforo-wrap header:before, #wpforo #wpforo-wrap hgroup:after, #wpforo #wpforo-wrap hgroup:before, 
#wpforo #wpforo-wrap menu:after, #wpforo #wpforo-wrap menu:before, #wpforo #wpforo-wrap nav:after, 
#wpforo #wpforo-wrap nav:before, #wpforo #wpforo-wrap section:after, #wpforo #wpforo-wrap section:before, 
#wpforo #wpforo-wrap ul:after, #wpforo #wpforo-wrap ul:before {
display: none;
}

Hey,
Thanks for the information, the solution and the quick answers!  😀  👍