I just got a set of 225/55/17 all around for winter tires (including steel rims) - however the back looks sooooooooooooo ugly.

Can i add spacers at the back? If yes, how many mm spacers should I add?

Also, will it affect driving condition?

Yes you can add spacers to the rear.
How large of a spacer is completely up to you. Without knowing the wheel width and offset nobody will be able to assist.
No it wont change the drivability of the car.

great! thanks for your info - that was quick... i'll buy appropriate spacers according to my specs.

You're welcome.

just for looks in the winter? then youre gonna have to make sure they do rust and get stuck to yor rim and /or hub.

If you're only using it for winter and such, you'll be fine. I've run spacers on my previous cars in winter (for winter only) with no issues) just make sure you clean it off well with good cleaning products and I lightly sand it too as well time to time once the season is over. I had 3 sets of spacers over the course of 4 cars and they all saw winter and never had issues. Just take care of them and you'll be just fine.

I got 225/50/17 for the winter tires.

What spacers do you guys recommend? Stock height.
I've heard 10mm on front and 15mm on back - is this good enough?

See Post #2. You'll need to post offsets and wheel width, not tire size.

Woh, this thread is from March! You sure were preparing ahead of time lol.

They're just the regular 17” Steel Wheels.

Are they standard for all of them? If not, where can I find this info?

Diameter of the wheel is not as important as the width and offset of the wheels. Is "Steel" a brand?

On some wheels (especially aftermarket) it's stamped on them or there's a sticker somewhere, or you can find it online. I doubt your steelies will have any of that. If I were you I'd just measure how much farther you want them to stick out and go with some conservative spacing somewhere around that thickness.