There should not be /css/ folder. The color.css file path is /wpforo/wpf-themes/classic/colors.css

I have that file. Looks like maybe there's a bug then?

The file that is 404'ing on me is defined in your wpf-includes/wpf-hooks.php line 956

wpf-includes/wpf-hooks.php:956:         wp_register_style( 'wpforo-dynamic-inline', WPFORO_TEMPLATE_URL . '/css/wpforo_dynamic-inline.css', false, WPFORO_VERSION );

This is not a bug, wpForo is installed on thousands of website and we've never seen such problem. This is a site specific issue. Is this a tst forum? Can you uninstall (remove all data from database) and delete forum? If so please d this and install again. Also make sure all caches are deleted when you're checking.

Ok sorry to insult by suggesting a problem may exist since your code is statically defining a file that does not exist on the filesystem. 

Not a test forum. I cannot delete the database.. 

This should only be defined and this file should be automatically created based on one specific condition. In 99% this file should not be called and created. The main reason is color.css file reading. If wpForo doesn't have read permission to colors.css it tries to create another using other ways.

Not a test forum. I cannot delete the database..

Then don't Uninstall, just deactivate, delete (remove plugin folder)  and install it again. This will not remove any data.