Non-Toxic Paint or Varnish for Pet Furniture

Hi! Sorry, I don't know where I should post this. Since it concerns about my cat's well being. I decided to post it here.

I have recently bought some crates (in Birch color) for my cats to climb and rest inside. I would like to re-paint or re-varnish it to darker brown color. Anyone can recommend the type of paint, spray paint or varnish which is safe to use on pet furniture?