You can't steal ownership of stuff yet, but Theft is useful for Upgrading/Breaking locks, better chance of success to make them, not sure if it affects breaking them. Stealth is good to hide from people on planet's surface as those have lower Perception than your Stealth can't see you.
There is no way to gain ownership of entities without either conning a player out of them when trading, or being the assigned pilot/commander when the owner dies. It is said that true theft will come with combat, but which stage we do not know. Likely to be a year or more though.