No Shadow Keeper for MAC, unfortunately.
You'll just have to use Near Infinity. First , backup the save you want to edit, open the relevant save, open Baldur.gam, and on the top left corner you'll see your party members' names. Place the cursor above the one you wish to edit and then click View/Edit. On the topmost of the new window, switch the View tag to Edit, and then scroll all the way down to "CRE file". Step on it and click View/Edit. In the new window you should see all the thieving skills and other categories you may want to edit. Change them to your liking. Close the opened windows, and after you try to move away from the Baldur.gam, it'll ask you if you wish to Save or Discard the changes. Choose Save Changes. That should do it.