Love this theme! For anyone who is wanting to use it with 3.6.12, here's how:
Please note I do not take credit for this method, it was originally posted on July 15, 2010 by Mushroom Kingdom (Another awesome theme!). I just modified the instructions to be a little more clear and applicable to this theme! Happy Holidays!
1) Open IE or any browser that isn't firefox.
2) Go to https://addons.mozilla.org/en-US/firefox/addon/9762/
3) Go to the bottom of page, under release notes select Version 1.1. On the next page click on the "Download for windows" button. Click download anyways. The theme will be saved as a .xpi file. To open it you will need Winrar.
4) Download and install Winrar if you don't have it already (google it if you never heard it before),
5) Right click on the xmas-1.1-fx-win.xpi file, select properties, and click change on 'opens with' section. Browse and select winrar to open these file formats.
6) Double click on xmas-1.1-fx-win.xpi and it should open in Winrar.
7) Double click the file install.rdf. In the file find the line that starts with:
Replace that whole line with:
7) Save the file.
8) Close Winrar
9) Start firefox and drag xmas-1.1-fx-win.xpi into the add-ons window, install, restart and done."