Xcode has had a plugin architecture going back to when Interface Builder was its own separate app. However, this system was relatively obscure, undocumented, and not widely used by third parties. Despite this, developers like Delisa Mason and Marin Usalj have done incredible work creating a stable and vibrant ecosystem of third-party Xcode extensions.
Simply install Alcatraz, and pull down all of the plugins (and color schemes and templates) that you desire.
Nice list of Xcode add ons.