@badge,
Insert the below CSS in Custom CSS Code field from wpForo > Settings > Colors & Styles Tab.
Please note that it's not a good one as there you have to set every forum separately, I'm sorry, but this is the only CSS that I could write for your request:
#wpforo #wpforo-wrap #wpf-forum-4 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-4 .wpforo-forum-data,
#wpforo #wpforo-wrap #wpf-forum-6 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-6 .wpforo-forum-data,
#wpforo #wpforo-wrap #wpf-forum-5 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-5 .wpforo-forum-data,
#wpforo #wpforo-wrap #wpf-forum-9 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-9 .wpforo-forum-data{
background: #f2f2f2;
padding: unset;
margin: 0px 0 10px 0;
}
#wpforo #wpforo-wrap #wpf-forum-2 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-2 .wpforo-forum-data,
#wpforo #wpforo-wrap #wpf-forum-7 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-7 .wpforo-forum-data,
#wpforo #wpforo-wrap #wpf-forum-8 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-8 .wpforo-forum-data,
#wpforo #wpforo-wrap #wpf-forum-10 .wpforo-forum, #wpforo #wpforo-wrap #wpf-forum-10 .wpforo-forum-data{
background: #cbcaca;
padding: unset;
margin: 0px 0 10px 0;
}