It took a bit of work, but I've gotten Poly Universe to the point where much of the content is moddable and the process for modding is (hopefully!) quite easy.
To create your own mod, you can start by downloading the sample here:
https://github.com/Pouchmouse/Poly-Universe-Sample-Mod
Place that sample mod in your local mods directory -- which is probably C:\Users\yourname\AppData\LocalLow\Pouchmouse\Poly Universe\Mods\Local. You can always check the 'Mods' menu in game to see the exact location of your local mods directory).
Now restart the game and go back to the Mods directory. The sample mod you downloaded should appear in the list of available mods. You can click it on and off just to see that it has an effect. It will change the opening theme music, replace the button click sounds with laser sound effects, and change the tree models to trees that are somehow even more low-poly than the normal ones. =)
Head back to the sample mod directory, and you are free to go absolutely nuts adding stuff. The info.txt file contains a complete list of the files that are moddable inside the game. Anything that you want to change in your mod, just include a file with that name inside your mod directory. When you have something you'd like to share, return to the game, find your new mod in the list, hit the Upload to Steam button, and it will appear in Steam Workshop.
Happy Modding!