In episode Hard Copy, the Dark Troopers were similar to Droidekas, being a ball with four thin robotic legs, two turrets on it's arms and a retractable cylinder head at the top. They were dark purple.
Later, in episode Dread 'N' Breakfast, seen Dark Troopers looked like more traditional Stormtroopers, no longer being robotic, with fully white armor, white gloves and helmets with blue "T" shapes on the front to see out of. They also have similar proportions to Dark Laser. Their appearances in this episode has been the one most commonly used since then.
More normally proportioned Dark Troopers are also occasionally seen, with white shirts, pants and caps that mimic the look of the more muscular, armor wearing Dark Troopers.
Their first appearance is in episode Hard Copy, where they came directly out of the Toy Catalog with Dark Laser and causing destruction all over Dimmsdale.
They returned again with their leader in episode Dread 'N' Breakfast with the updated design, staying at the Turner's House when it was turned into a Bed and Breakfast so that Mrs. Turner can make extra money without know that they were trying to destroy Timmy.