Surfing the forums just came on my mind that SMF and now ElkArte does not handle categories in a modern way.
So my suggestion is:
1.Add category view - When you click on a category it will not collapse it (its the way it works atm.) but open it as new page with all boards that belong to it. (Just the same thing as board index only show one category and not all)
Same thing with link in linktree...atm if you click on category name it brings you to board index and scroll to requested category
All modern forums support it (free amd paid)
2.Option to set theme for each category
1) is basically what SleePy's Single Category Mod does, shouldn't be that hard to implement.
http://custom.simplemachines.org/mods/index.php?mod=486
2) There's currently an option to set a custom theme by board, not sure if we need it for categories, too.. I think it's a rarely used feature at all, I have personally never seen a forum using different themes per board.
Styling categories and their boards can also be done via css, quick example:
Its not hard to implement at all since board index already exists and category view can be done by ISOLATING requested category and maybe hiding info center...Its not hard at all i made that mod myself long time ago for my game server community and as i remember it was piece of cake...
About custom themes for boards/categories. Personally i did not see much use of it either...but it can be very useful, instead of installing blog mod we could create theme with blog layout and use it to "simulate blog" (Just and example, its not smarter than installing blog mod). Bigger sites could use separate themes to "split" forum and give it special touch... Its simple thing to make and its just few variables and checks so it wont slow down the forum...