If you come across a Tumblr theme which isn't in Tumblr's theme garden, you'll probably have to install it via copying and pasting the code. Here is a quick and easy tutorial on how to do that.
1. First you need to go to your dashboard of your Tumblr. Then click "Customize Theme".
2. You'll then be presented with the theme edit screen. Click on "Edit HTML".
3. You'll then see a bunch of code. Select it all and hit delete. It should then be completely empty like this.
4. You then want to get the text file which has all the code.
- Windows users - Make sure to open the file with "Notepad"
- Mac users - if the code doesn't look like what is in the screenshot below, open the file with TextWrangler
Copy all of that code and paste it into the Tumblr bit.
5. Make sure to hit "Update Preview". Then a "Save" button should appear next to the "Update Preview" button, hit that "Save" button.
And there you have it, your new theme should be installed!