Hi Stephen,
I just updated to bbPress 2.5.4 and noticed the css load issue again. I’ve created a patch and my first trac ticket here!
https://bbpress.trac.wordpress.org/ticket/2613
Solved:
In Windows bbPress was reporting $content_dir with double slashes // (ie: c:/www/wordpres//) so the content_dir was not being properly string replaced with the content_url()
Solution:
Line 168 & 237 of template-functions.php added this to correct for double slashes:
$content_dir = str_replace( ‘//’, ‘/’, $content_dir );