Wouldn't a good view be tabs with "Add-Ons", "Graphics", "Styles" as headers then throw all of the addons under "Add-Ons" and so forth? That way you're at least at the top level of the hierarchy when you start out and you aren't presented with what looks like a jumbled mess.
This isn't criticism in a bad way, just trying to make it better is all and nothing meant negative towards the work already done!
The list is very visually cluttered, confusing and frankly ugly.
The list would look infinitely better being categorised or contained within drop downs slide out menus in the first place, if one is after a Style they don't need to be assaulted with a confusing list of Add-on Categories to wade through before finding that styles options.. likewise, if down the line you add further categories for Add ons or Styles or whatever, the list just gets even more confusing and ugly... I know it is an extra click, but probably worthwhile.
I also agree with whomever suggested the list be on the right, consistent with XF design.
ATM it is not looking as stylish as the rest of the forums, which is disappointing.