I know this reply is super late but I had the same question earlier today because I absolutely hate the new artstyle, and in fact you can! I looked through game files and code and everything is still there. The dev has just added some band-aid solutions to try to hide it.
Basically all the code and all the assets are still there, he's just removed the button for toggling and added a line that toggles back the old art every time you go to sleep. If there's any demand for it I could write a guide on how to fix it or maybe upload a modded version with the toggle button.