Another years late post.
Suppose you want to convert the blue Dojo Toolkit theme to another color. How you do it? Run this command of the ImageMagick suit on all the images in the src/widget/templates hierarchy
convert -modulate 100,100,45 <blue_image.xyz> <green_image.xyz>
This rotates the color wheel by 45 degrees and keeps the other parameters constant. The result is a light green theme. You have to update all the css files too and that’s way more time consuming. However, with a simple conversion can applies to HTML colors with this particular color rotation : just convert all the occurences of #rrggbb into #rrbbgg!