When I built my PC I did not want any of that RGB stuff, but the case I landed on had a glass sidepanel and it would be a shame if everything was just pitch black inside. So I decided to turn it into a magical terrarium.
Here is how I did it. The dragon fly is cut out of acrylic sheet on a laser cutter, which also etches the markings on the wings and tail. To do this I drew it in Inkscape using two different colours. In the laser cutter software you can then assign different profiles to each colour.
Then I used hot glue to insert an LED into the hole, which I covered up on the bottom with some black tape. I then cut one leg of the LED short and soldered a resistor to it. The value of the resistor depends on the voltage and the type of LED. You can of course calculate this but the YOLO way is to just do what the shop says where you got them.
To connect it I browsed the motherboard manual in search for a header with 5V, and in my case there was an addressable LED header of which I just left the data pin disconnected. I then routed the wire so that the dragonfly hangs nicely in the middle of the case.
Here is how it looks with the case open.