Bright suns, Nicholas! Thanks for bringing your question to planDisney.
While I think you could probably wear your Sith tunic and cloak on a party night, which features different
Costume Guidelines, a regular park day can be trickier to navigate than the Kessel Run in what could look like a costume. Unfortunately, your mileage may vary as to whether your outfit will be deemed acceptable or not. I spoke with a Cast Member and they couldn't provide me with a firm answer, because it's always up to security what can safely be permitted.
Even on a party night, I cut my child's Mandalorian cloak shorter than it had been for Halloween for safety concerns since it was almost floor length before the chop. Park rules are clear on tripping hazards. Depending on how much the tunic looks like a costume, however, that could potentially be okayed for entry. Never tell me the odds!
We usually get a little creative with our attire to approximate our favorite Jedi or Bounty Hunter looks without actually donning costumes to avoid this issue. It's sometimes more comfortable too. But if you want to try to enter the parks wearing your gear, I suggest you have an alternate shirt to throw on in case the tunic isn't allowable. Just throw it in a backpack and be ready to adapt. Seems like something a Sith could manage with ease.
May the Force be with you.
Elizabeth