Edit category settings
From MoodleDocs
Moodle1.9
Location: Edit category icon in Administration > Courses > Add/edit courses
In Moodle 1.9 onwards, a description can be given to each course category. If category themes are enabled (in Administration > Appearance > Themes > Theme settings) then a particular theme can be set for all courses in a category. The theme will also apply to sub-categories and the category page itself.