First thing I would do is get it off that substrate and use paper towel. What are the temps like? The dark coloring makes me think it's not warn enough. If it is cold, that can contribute to it being skinny. Although you do say it eats well. What is it eating and how often? It's possible that if it's eating and not gaining weight, there could be a parasite issues. I'm going to move this to the leopard gecko health section where it will get more attention.