Really fun, expansive and challenging game!
Along with general snazzing up of the aesthetic, the only real gameplay change I would suggest is implementing something which meant the player wouldn't need to reload for ages upon death, as that got a bit tiresome.
There were also a couple of enemies who I felt were a bit of a push-over, at least early on, so a little more work on balance could help make those first levels all have a good punch.
Nice work though!