I have been caught stealing once. I think someone saw me steal a spare change of clothes at The Winking Skeever in Solitude after I traded all my belongings with Malborn on the mission Diplomatic Immunity. I never recieved a bounty or any nasty comments from NPCs as I left the inn.
Shortly after the quest, thugs were sent after me and I dispatched them and found the bounty letter written up from someone from the inn.

What dictates being caught stealing?
What are the repercussions of being caught stealing?

I ask because I am currently working on the College of Winterhold questline and seeing as there are rarely any NPCs in the Hall of Attainment I want to rob my fellow mages' rooms of their valuables before I leave.

- My sneak is 100.
- I heard that if you steal/aggravate someone enough they will send a Dark Brotherhood assassin after you.
Thanks in advance!

If you steal, there's a chance they will hire thugs to teach you a lesson. DB assassins only come if A)you aren't a member of the DB and B)you killed somebody important to a certain NPC

Thanks!

I hadn't killed anybody on my character before he turned bad, and had 4 contracts on my head.

Really? How? Every time I've had DB assassins come after me, it was always linked to somebody I had killed, but thugs would only come after me when it involved a theft(surprisingly, bandits can send thugs after you as well even if you slit their throat right after stealing from them.)