Alright, well, I can do that much.
Just find the theme, download it, and when you open it up you should find 2 things. A .xml file, as well as an images folder.
In your ACP, select the Templates and Style tab at the top. From there select Import a Theme. Then, under Local File, you should see a button that you can click to find a file. Pick the .xml file that you had in the folder for the theme. Enter whatever you want it to be called in the New Name section, or just leave it blank for the original name. Click the box beside Ignore Version Compatibility then hit Import Theme.
After that, you'll have to upload the images folder to your ftp. Put it in MyBB Root/Images/ and your theme should be working perfectly. (MyBB Root being the root of your forum system)