Because I have seen nothing on these forums about how to use a theme on mac I have figured it out on my own. So first you need to open up the mod folder, you then go to view and press the "show path bar"  after this you go back to the data folder, then themes folder, and finally but your downloaded theme into there, you then go to interface and themes, and finally choose your theme you've downloaded.   I made this post because i have not seen a guide anywhere else, I hope this helps