This has bothered me for years, and even after visiting Star Trek: The Experience back in 2008, I still can't figure out the answer. On the port and starboard walls of the bridge, directly aft of the dedication plaque and bridge replicators, are a series of beige (or brown, in season 1) panels. In season 1 they look to be made of wood, and from seasons 2-7, they appear to be...aphostered? I'm not sure. It makes little sense to have plush walls, even if the texture is leather or pleather, but this is, after all, the Enterprise-D we're talking about. If anyone can help identify the materials these panels are made of, let me know! Thanks!