Am I allowed to use the default CSS file (Themes/default/css/index.css) for things other than the forum? If I cited SMF, of course...
I am not sure what you intend to use it for? It really would be no good to use for something else since it is designed to correspond specifically with certain SMF files. ???
I am just using it like for this file: www.epicpwnage.co.cc/404.php
make it look sorta like SMF so the design stays somewhat constant
Anything SMF needs to have the SMF copyrights. Since your design is not SMF but only made to look like SMF then I would think no, you are not allowed to use the SMF CSS files for the application in question. Without copyrights you really shouldn't be using the SMF images either.
If you are just using it for the 404 page then you still need to have copyrights in place.
http://www.epicpwnage.co.cc/404.php
;)