Considering how difficult the game is, there are scant few cheats available for this sneak 'em up. However, we did manage have come up with these:
If you're stuck on any mission press Ctrl, Alt, Shift and End simultaneously to skip to the next one.
Create a level select by adding the line "starting_mission X" in your 'dark.cfg' file (X is the mission number). When you select New Game the game begins on your specified stage.
If money's a problem for you, add the line "cash_bonus" to the 'dark.cfg' file in the Thief directory on your hard drive.