Hi, Rob,
The category bodyclass is added automatically without having to enable anything. I think David got that wrong!!!
To check just see the source code for any page in Moodle and the category will be present...or should be.
For example in this forum page you can see the category.
<body id="page-mod-forum-post" class="format-topics path-mod path-mod-forum gecko dir-ltr lang-en yui-skin-sam yui3-skin-sam moodle-org pagelayout-incourse course-5 context-115 cmid-46 category-3 two-column has-region-side-post used-region-side-post side-post-only jsenabled dndsupported">
Cheers
Mary