It should be one point of treasure. Rules say "steal 1 treasure" I think because they didn't want to use the word "point" in order to stick to the theme. "Steal one point of treasure" would have been a better wording. Thanks for the heads-up!
My brother takes the ninja theme very seriously, and literally thought it meant steal treasure token, when no one was looking attempted to steal a gold token from another player. Had to let him know that it is as Rocco Explained, one treasure point, aka copper piece.
I do think future editions of ninja dice, or expansions could include some dexterity mechanics for how dice are handled when thrown(aka, penalty for knocking dice, boon for catching a die which was about to roll off the table etc..), and or fumble rules to go along with the ninja theme. Or maybe just home rules.