Higher the Accuracy, higher the chance to hit your enemies, both with weapons and spells. IMHO, every character should have at least decent value.

I'm fairly positive that Thievery is not affected, but I don't have any official source to back it up.

Accuracy is the stat checked for shooting. It probably affects your chance to hit in melee and maybe your chance to hit with spells or thievery. It's still unknown what exactly this stat does.
Accuracy is easier to raise in MM3 than in WoX.

For a thief or ninja acc is less useful than luck and about as useful as str (especially useful for the two characters in front who bash walls), end (you need HP for surviving traps) or spd (hard to raise). Which stat is more important depends on the rest of your party and which role this character is supposed to play, just try to get all stats except int and wis (which are useless) high.

For the ninja or thief role I recommend a dwarf in MM3. They get the highest racial thieving bonus and more hitpoints.

Thievery depends on the character type as well as race. For MM3, Elves and Dwarves both get a +10 Thievery bonus(best bonus to the skill). I do know that elemental resistances and possibly Thievery racial bonuses get changed a bit for Xeen.

Accuracy helps determine whether or not you hit a monster with a weapon or spell when you attack. Accuracy is a prime statistic for Archers and Ninjas. Speed is a secondary prime statistic for Ninjas. A Ninja will need a score of 13 at minimum for Speed and Accuracy.