Prada Sunglasses Frames Turned Yellow and Store Refuses to Address the Issue

Last year, I bought a pair of Prada sunglasses with white frames. From the moment I tried them on, they sat crooked on my face, so I went to the store three times to ask if there was a problem. Each time, they assured me there wasn’t. Then, the white parts of the frames began to turn yellow. I sent them in for inspection, but after a long wait, the store told me it was "user error" and returned them to me. This is definitely not a user error. I paid a premium for this brand, and the quality should not be like something bought at a bargain market. The store staff are also incredibly unhelpful and seem clueless. I need a resolution for this issue.