This is not wpForo issue, this is a conflict with your theme header CSS, so these are totally different cases. For the last website you should use this CSS code to fix the conflict:

body.wpforo  #wrap_all{background-color: #ffffff!important;}
body.wpforo .av_minimal_header .header_bg {opacity: 0;}

Put this css code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea, then delete all caches and do Ctrl+F5 on forum front.

That did the trick!  Thank you for resolving the conflict!