Do you mean decorations or do you mean the sky/barrier in the background?
Here’s a guide I found over sky/barrier backgrounds
I’m not really sure if any of those are your style or will fit with the games theme though, if you tell me the theme you’re going for I can try to and make one (and send you steps on how to recreate it). The main thing to making a background like that is by using barriers and changing their color/transparency to your preferences