adding background to Transparent Stock image

I have a stock image I downloaded from adobe stock: Minimal Nashville Linear City Skyline with Typographic Design - Buy this stock vector and explore similar vectors at Ado…

I want to make something like this: Atlanta Skyline Invitation

The issue I am facing is, how do I make the skyline itself not transparent? If I put a black background on it then it all goes black because there is no fill to the skyline but when I open it in illustrator there isn't a way to change the fill for the individual buildings.

Please let me know if this is possible. Thank you!