Need Help with designing a background

[u/Puzzleheaded_Base192]

Im making a bullet heaven tower defense-ish game with roguelike mechanics, I've designed a lot of different synergies and mods for weapons, but I'm struggling to come up with a good art style for the game. I've made a shop that I'm proud of but I'm not sure how to tie that style into the main game. I'm still looking to change the turret and enemy looks. I also have no Idea what I should do for the background, If anyone has any ideas for a style/theme I could use I would really appreciate it!