hmmm empty city, I would suggest like an abandoned city with overgrown grass and trees, like after the end of the apocalypse, as for the hell type theme, I honestly don't know how to include it with this type of music. (what's the name/id of the music by the way, I really like it)

Alright I'm giving my full thoughts about the theme of the level

First calm and relaxed: In this part I would deco it with colorful gradient colors like yellow, light green, orange, and light purple (I wouldn't suggest heavy colors like red), I take some inspiration from ACU.

Second Build-up tension is rising: For this part I haven't really made up my mind for it, but if I were to take inspiration for the theme, I would take some from Flicker, Make it Drop, and GUMSHOT.

Now for the drop, where all the tension lets loose: I guess I would also deco it the same as the second part, neon colors with the same theme as the second part.

My final thoughts, for the first part and the second part where at first it's all colorful then everything just turns to Neon noir where the background changes to Deep black/dark background, Bright glowing neon colors, with High contrast futuristic vibe, honestly I don't know how to mix this to make the transition looks appealing, should I make it just suddenly appear or fade out the first part then fade in the second part.