I created a nice, professional-looking green theme by modifying the gray theme that comes with ext.js 2.0.
To create the theme: I colorized the images using Gimp's colorize function set to hue=159 / saturation=86 / lightness=11. To change the css, I created swatches for the colors in xtheme-gray.css and colorized in Gimp, then replaced the colors in the css. The download includes the swatch sheet for the gray theme.
The code is at http://tobydietrichlibs.googlecode.com/ - look for the extjs-green-theme download.