I am using 5.3, no intergration.
In default.css, the .menubarnb properties seem to be being ignored. I went in to indexhead.tmpl and temporarily changed a "class" statement with a hard-coded bgcolor and that displayed the hard-coded color.
If it would help you can display http://www.remarkablecars.com/photopost/index.php
. The PhototPost is using default.css. I do have a headtags.html page being loaded that includes another .css page but I have tried this with and without this include.