Well, your tileset is very generic at the moment. It also feels like you rotated one block (either vertical or horizontal) to get every angle you needed, which makes it feel very unnatural.
The rotated tiles were for a placeholder while I worked on the real angled tiles. Do you have any suggestions to break up the monotony of the tiles? I've never done cave tiles before so I'm a little unsure of what direction to take.
edit:
Alright, I finished the angled tiles:
Just a thought but I would shoot for a multi layered background, something that scrolls at different speed to the foreground or even 2 layers of the background that scroll at different speeds to each other an the foreground. That way you can do simple 'fillers' but they look a million dollars.
I believe this is what you meant doing? I altered the original sheets colors and then made three other copies with darker hue-shifted versions of those tiles to be used in the background. I think my tiles were really helped with the addition of the more natural looking slopes as well.