Ciao Luciano,
Si certo...
If you checkout the 'source code' for the page like I did here you can find the 'body class' of the category.
In this forum it is category-3 see here...
<body id="page-mod-forum-discuss" 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 has_dock has_dock_left_vertical">
So you can set these out in your theme CSS like so and there is no need to add body.category-3 { background-color: red } as these CSS selectors are already body classes.
.category-2 { background-color: red }
.category-3 { background-color: orange }
.category-4 { background-color: yellow }
.category-5 { background-color: green }
.category-6 { background-color: blue }
.category-7 { background-color: indigo }
.category-8 { background-color: violet }
Hope this helps...