Tweed is a fabric most often associated with the prim-and-proper—the Coco Chanel’s and Blair Waldorf‘s of the fashion world. It’s classic and polished, often found in matching blazer-and-skirt sets. For Fall 2018, though, designers did a 180, showing tweed in a brand-new—and decidedly non-preppy—light: with metallic shimmer (at Moschino), neon threads (Prada), and, in the…