Many administrators often differentiate categorie in Moodle by using Category themes. More often than not these are just variations on a theme with either a different color scheme or logo. However to implement this involves duplicates of themes to be added to the server and adjustments to be made (typically using parent themes).
With the new theme settings options in Moodle 2, this could just as easily be done now with theme settings. Organisations could also then build their own themes with options in place for exactly what they want changed per category. This would alleviate a lot of code work in a large proportion of cases.
I am suggesting this be allowed at a course or category context as these are obvious locations. Once a theme was set for a category, I envisage a simple "Category theme settings" or "Course theme settings" link to show in the settings block to facilitate the changes. Changes are then stored alongside a cat or course ID for reference.
May want to include new options either in "Theme Settings" to allow suers to customise theme's as a simple Yes/No menu toggle, or for new permissions to be added that can be given to the roles we want give this ability to.