@zeeshan333
The style conflict comes from your theme or from other plugins. I can't fix it 100%. However, this CSS code might help you.
Put this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea, save it, delete the Autoptimizer cache, navigate to forum front-end and press Ctrl+F5 to reset browser cache:
#wpforo #wpforo-wrap .mce-container,
#wpforo #wpforo-wrap .mce-container *,
#wpforo #wpforo-wrap .mce-widget,
#wpforo #wpforo-wrap .mce-widget *,
#wpforo #wpforo-wrap .mce-reset{vertical-align: middle !important; font-size: 15px !important; color:#666; letter-spacing: 0px;}
#wpforo #wpforo-wrap .mce-container-body .mce-btn button{padding: 3px 4px!important;}
#wpforo #wpforo-wrap .mce-container-body .mce-btn button:hover{border: none !important;}