1) Copy this entire code.
2) Open up the customise.
3) Click on Edit HTML
4) Delete the code already in there.
5) Replace it with this code.
6) Hit Update Preview, and then Save.
7) Refresh the page, and press Save again.
8) If it wont let you paste in the box, or if it still does not work, use classic customize. (http://tumblr.com/customize-classic/)