You can't "open source" the theme actually; i mean the developer is asking a small sum of $1.66 for giving you the files to use. Themes on the n9 and I'm sure n900 too are just a collection of folders containing modified stock theme files...

Once you buy it from the developer you have the files and can do to them whatever you wish to modify/change them...What is there to open-source in them?

I would consider the small sum in fact lesser than what could be considered as a donation to a developer who posts his theme here as free for all...

The effort required in creating modified .css files for the n9 is real huge so i would suggest to the OP to pay that amount instead of trying to reverse-engineer it; its the least the original developer deserves for his work...