So I followed the instructions at WordPress to change the bbPress editor look, and match my dark theme (as posted above by netweb) and it didn’t work. I’m not sure that page is current for the new WordPress. Not only did it not change my editor’s CSS, it gave me a php header error that wouldn’t let me access some pages.
I also tried to put the CSS in the bbPress css in my child theme, hoping if I narrowed it down enough it would take precedence, but no luck there. Also, there are several buttons involved in the editor that are controlled by css in yet *another* css file (located in the same folder as the editor: wp-includes/css) buttons.min.css that is also causing a ton of white in the editor, and when I tried the php fix to create a custom css file for that one in my child theme the whole site crashed.
It seems really strange to me that such a huge part of this plugin doesn’t have a clear solution for those who prefer a dark theme after all this time. I’ve literally spent hours today trying to problem solve this. The glaring white looks really, really bad in my forums. If I ever sort it out, I’ll do my best to make sure and get the directions out there.