I think that you're going to need Sencha Cmd 3.1 which knows how to configure and build Ext 4.2 themes. Unfortunately the final Cmd release corresponding to Ext 4.2.0 has not yet been released -- I understand it's expected to release very soon.
Some discussion here: http://www.sencha.com/forum/showthre...or-ExtJS-4.2.0
Once you have Cmd 3.1 you'll be able to set the app.theme config property in .sencha/app/sencha.cfg to specify the theme, or do so at the command line. Here's the current output of 'sencha help config':
You can probably get more specific help in the Ext forums, since this isn't specifically related to Architect.
This command can be used to either set configuration options singly or load a
configuration file to set multiple options.
Because these configuration options are only held for the current execution of
Sencha Cmd, you will almost always use then to chain commands that will now
be executed with the modified configuration.
For example, to change the theme of an Ext JS application for a build:
sencha config -prop app.theme:ext-theme-neptune then app build
Multiple properties can be loaded from a properties file:
sencha config -file neptune.properties then app build
The content of "neptune.properties" might be something like this:
* --file, -f - Specifies a configuration properties file to load
* --prop, -p - Specifies a configuration property and value to set