A quick Google search for the city name of interest + skyline will turn up a wealth of results.
Another search for clouds + stencil leads to many options for background clouds that make a beautiful backdrop for the city.
If you're lucky enough to have access to a laser cutter, you can cut out your skyline that way.
Alternately, you can print or trace your skyline on heavy construction paper and cut it out using scissors and/or a hobby knife.
The circuit for this project is wonderfully simple to build, thanks to the alligator clips already attached to the NeoPixel strip.
Connect the Black clip to GND, White to A1, and Red to Vout.
Tape NeoPixel strip, Circuit Playground Express, and battery pack to a long rectangle of cardboard.
Glue clouds onto cardboard base.
This will serve as a backdrop for the city, reflecting light from the NeoPixel strip.
Once your clouds and skyline have been glued in place, you should be ready to turn on power to the Circuit Playground Express and experience your illuminated city for the first time!
You can test that it responds to ambient light conditions by moving your hand over the light sensor on the Circuit Playground Express board and watching the NeoPixel strip dim as your shadow passes over (when slide switch is in the Left position).