For anyone interested,
I have found a way to run "compass watch" on themes generated by "sencha generate theme".
Here are my steps:
 Download and install latest Sencha Command (Sencha Cmd v18.104.22.1684)
 Download and install Ext JS 4.2.0, and unzip. (in my case I unzipped to "ext")
 cd ext
 sencha generate theme ext-theme-mytheme
 cd packages/ext-theme-mytheme
 sencha package build
 Go get coffee
 cd build (you should see ext-theme-mytheme-all-debug.scss and such)
 because compass only checks the timestamp on ext-theme-mytheme-all-debug.scss, and because this file doesn't actually change, you have to kick compass to re-read that file and generate again. This can be done by
compass watch --trace --force --config ./config.rb --sass-dir . --css-dir ./resources --images-dir ./resources/images
Windows users don't have the "touch" utility so:
 now the fun begins. From this step on you have to dig into a theme as see how things work. Change parameters, then touch the scss file, and compass will regenerate.
copy /b ext-theme-mytheme-all-debug.scss +,,
 After three weeks, when you are done tweaking, then rebuild the package and you have a working theme.
I hope this helps a little.