I finally discovered how to customize a theme:
The sample application 'Ordering' use config.ui ="blue-panel" in the advanced part of any forms to change the aspect of the form. But, what is this?
ExtJs use the css class '.x-panel-default-framed' to paint a form.
Config.ui="blue-panel" change 'default' by 'blue-panel' so the class used to paint the form is '.x-panel-blue-panel-framed'
So, you have to make a class that defines the custom form, e.g.:
.x-panel-myclass-framed {border-style:solid;border-color:green; background-color:red;}
and use config.ui="myclass" in the form you want to change.
It is simple as you know how to do it.
There are more classes that can be changed. You can use 'developer tools' of chrome to investigate.