In Schoolism, we learn about placing characters/subjects into a background. I have ALWAYS wondered how in the world do people place complex characters into complex backgrounds and make it look unified. Besides obviously painting the whole image at once together... here were some tips:
-overlay some of the background colors onto the character
-use correct perspective
-unify all the shadows and lighting of the character and subject
-remember to put a cast shadow to solidify the character onto the ground
-things further away are less saturated and less detailed

Here is the character I made on the side: https://i.imgur.com/kTZqWo3.png
And above you can see the color lighting changes I made accordingly. It's not a huge difference, but I think the baby steps make it look pretty ok.