Hi gwood333,

please specify which part of topic background you want to change?

Like your message. I want to make your message the background color black. Of course, we can change the text color to white.  How do I do that?

Thanks.

Please use the Dark Style. Navigate to Forums > Settings > Styles and choose the last "Dark" option. Save it. delete all caches then do CTRL+F5 on forum page to load new styles and colors. It'll look like this:

If you want to change it separately, you should use the 9th color option in color list. But this color is used for many items, not only for post background.

You can also use custom CSS, this code should be put in Forums > Settings > Styles admin page "Custom CSS Code" textarea:

#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content { background-color:#666; color:#fff;}