Well, if someone stole from me in the open, and I was a werewolf, I'd probably be pissed enough to attack them, too.
First option would be to simply load an earlier save, from before you stole the item.
If that's not an option, then two things:
1. Learn from this and make multiple save points so you can recover if you make a mistake. RPG players are notorious save-spammers specifically so they can reverse mistakes like this. Save-spamming is your friend.
2. Try this to fix it:
a.) Make a new save for your game, so if there are any problems caused by using console commands, you can restore your game.
b.) Leave the Frostmoon Crag area. Go somewhere you know the faction isn't. Heading back to Windhelm would work perfectly, but Raven Rock might be far enough outside their possible cell influence to work, as well. The important thing is not to have a member of the faction in the area when you use the console commands, or it won't affect them.
c.) Open the command console.
- Now, before you read the console commands (which are below) read this:
From UESP on ID codes in Skyrim:
The first two digits of form IDs found in add-ons are given as xx because they may vary depending on the number of active add-ons and their load order.
Objects from a specific add-on will generally all have the same two leading digits. So, if Dawnguard is the only add-on you have, and you're not loading any mods, the xx for Dawnguard IDs would be 02.
The specific code is not displayed in the "Data Files" screen, nor in the Creation Kit, but mod managers and other utilities will often show them.
From within the game, you can find the correct ID by opening the console and clicking on an object from that add-on, or by using the help command with an add-on specific object.
So, below, you'll notice the faction number for Frostmoon Crag is "xx01DC61". That "xx" will need to be changed. Follow the instructions (above in quoted text) for finding YOUR Dawnguard's "xx" prefix and replace the "xx" with that number when using the FormID (xx01DC61) to add yourself back into their good graces.
Console Commands:
player.addfac xx01DC61 1
or
player.addtofaction xx01DC61 1
(The "1" tells the faction you're an ally.)
I hope this helps.
