The CSS code looks correct. 

Is there a way to disable the ability to attach and send files in wp foro private messages addon? A few days ago I got a reply on the main forum wp foro, but this way does not work: 

I may suggest you hide the section using the CSS code below: 

#wpforo #wpforo-wrap .wpforo-messages-content .wpf_attach_button_wrap {display:none;}

The CSS code should be inserted in the "Custom CSS code" textarea of the "Colors & Styles" tab within the wpForo Settings page. 

Now the "Custom CSS code" textarea looks like this:

#wpforo-wrap{
font-size: 13px; width: 100%; padding: 10px 0; margin: 0px; 
}

#wpforo #wpforo-wrap .wpforo-post-content a {
    color: #ED9300!important;
}
#wpforo #wpforo-wrap .wpforo-post-content a:hover {
    color: #ED9300!important;
    text-decoration:underline!important;
}
#wpforo #wpforo-wrap .wpforo-messages-content .wpf_attach_button_wrap {display:none;}

Is it OK? Thanks in advance!