My present thief design "steals" gold as a gameplay mechanic in that she actually just marks an enemy Champion and, if she can land a basic attack on a marked enemy, gets a good amount of bonus gold. Since you can chain several marks in a combo, you could earn something like 40 gold off one trade!
The downside? She's squishy and takes a huge gamble when she wants to trade. Someone like Darius would absolutely destroy her if she tried to pick his pocket (in-game).
So she doesn't actually steal gold out of another player's coffers, as that's toxic. But she gets bonus gold for being aggressive. Gameplay vs thematic, and I tried to make a compromise.
As for other stealing, her ultimate lets her steal a large amount of AD from an enemy Champion and heal herself for a large amount of her target's maximum HP, but ONLY if her ultimate is the killing blow (ala Darius). Thankfully, the attack does bonus damage based on % of the target's missing health, so it works well as an execute. (Does that sound too assassin-y?)
If you want more context, how about you give my thief, Severa, a looksie?
http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/gEIw1dEE-severa-the-princess-of-thieves