From the help file:
Options  * --controller-name, -c - The name of the default Controller
  * --library, -l - the pre-built library to use (core or all). Default: core
  * --path, -pa - The path for the generated application
  * --theme-name, -t - The name of the defualt Theme
  * --view-name, -v - The name of the defalut View
when trying to use it:
>sencha --theme-name=bb10 g a MyApp  d:\path
[ERR]           No such property : theme-name