Editing themes in Espresso is still more of an art than a science. It's not very well documented and can seem a bit buggy at times.
This video is mainly in intro for anyone looking to create a theme. It doesn't go too much in to detail, but should get your feet wet enough so you can start experimenting on your own. I cover porting a theme from TextMate, which is what most request are for it seems.
If you have any questions, post them in the comments and I will do my best to answer them.
Hope you enjoy!
Original Monokai theme can be found at http://www.monokai.nl/blog/2006/07/15/textmate-color-theme/.