Thanks! That helped a lot. I've found service_head.tpl file (under skin/common_files/customer/) and I can definitely see that's where this business is done.
My problem now is actually loading the files properly. I don't think I quite understand the syntax of the X-Cart short code language.
In order to pull in this file as I normally would...
<link rel="stylesheet" type="text/css" media="screen" href="http://mysite.com/custom.css" />
... do I need to put brackets around the statement? Or is it necessary to phrase it like this...
{load_defer file="http://mysite.com/custom.css" type="css"}
?
Actually there's not that much code in my css file -- could I just insert some CSS straight into service_head.tpl? If so how would that be formatted?
Thanks again for sharing your expertise and suffering through my newbness!