Sith troopers were the foot soldiers of the old Sith Empire... the one from Knights of the Old Republic, during the Jedi Civil War, with Darth Malak, not the other Sith Empires. They were the stormtroopers of their day. If you didn't have Force powers or a college diploma, you usually ended up as a Sith trooper.
The Sith trooper uniform was designed according to the following logic: mirror sunglasses are cool; therefore, mirror visors must be cooler; therefore, mirror visors surrounded by a shiny silver helmet must be even cooler; and therefore, armor mostly made of gleaming silver must be the coolest thing ever. The designers then went on to suggest spangles, rhinestones, and mandatory body glitter for all troopers not currently in uniform. The designers were then executed as a warning to the fashion industry. Intimidating though the Sith trooper uniforms were... they required an awful lot of polish, and even a mild food fight was enough to ruin them.