I've had a silicone case on my black iPhone 7 since I bought it in December, and I've noticed there are a load of "marks" on the back of the phone. I've tried cleaning them off but it doesn't appear to have made any difference. I'm a bit pissed because the idea of having a case on the phone is to keep it in as pristine condition as possible, to help retain value, and because I like to look after my stuff. However it looks like the case has totally borked the back of my phone. Pics attached - any idea how to get rid of these marks, or anyone else had the same issue? Looks like dirt has got stuck between the case and the phone, and scratched the back.