Sith apparel was the term given to the general attire of members of the Sith Order in all its forms.
Unlike the Jedi who did not believe in material possession, the position of Sith Lord was highly individualized and the personal attire of each Dark Lord reflected this.
The technology itself had known fundamental improvements in the previous decades, and gave a more austere and high-tech look to the whole Galaxy.
This is probably why individuals such as Darth Bandon, Darth Malak or even Revan himself wore such unadorned sets of clothes.
They replaced their red and yellow uniforms with azure blue/gray jumpsuits, with higher ranking officers wearing caps.
This uniform also came to be worn by the trainees at the Sith Academy.
The Jedi developed a similar garment which was equipped by their military forces.
Some of those artifacts were still intact and usable at the time of the Old Sith Wars, such as Aleema Keto's or Crado's robes, The Sith armors traditionally consisted of metal plates worn over dark robes, and were usually covered in sharp spines to be used as weapons.
These robes shared some similar fashion features with the Jedi Adventure Robe and the Ceremonial Jedi Robes.
At the time where Revan founded his new empire, the Sith likely chose to get more practical robes.
Most Sith Lords would feel pleasure and a sensation of power through the jealousy and greed their material possession aroused among their peers.
This attitude was still present amongst the Sith millennia later—Master Qordis was known to wear only the finest silks and most expensive jewelry, and he possessed luxurious apartments on Korriban.