Switch to folder in theme based on device without plugin?

Hello everybody,
I’m writing a new theme for my website and I’m looking for a way to redirect to a folder in my theme based on the device the visitor views my website, where each directory would have their own css and images. Here’s a visual of what I’m trying to accomplish:

Index.php
|
|- (desktop) <theme directory>/desktop
|- (mobile) <theme directory>/mobile
|- (tablet) <theme directory>/tablet

I’m sure there’s php code that can accomplish this but I don’t know where to look. I don’t want to use responsive as the CSS file is at 30K and want it to be as small as possible.

Thanks,
Gregory S.