Um, you're asking two different questions there... if you want to create a custom theme, there is documentation here and dozens of related questions on this site already. (If you know PHP it's incredibly easy to make a theme.)
As for how long it took to build... well the project started over a year ago and the first public version was released 4th February 2010. Gidgreen (the developer) spends a month or two on each version then takes a few weeks off before doing the next version. So it could be anywhere from 6-10 months solid work to get to where it is today.