Build application with client custom theme

I've an application with a default theme (less files defined in app/sytles and included in app.less) and reachable at my main website (for example http://www.example.com)

Now I need to build the same application for different clients. Each client has its own style customization (client.less file) and its own assets (images, fonts, ...). This site will be deployed at, for example, http://ift.tt/1sOAGeu.

My idea was to customize the build process passing an additional flag to the ember build --environment production command and customize the build process adding the client.less file when needed. Something like ember build --environment production --theme client1 but as far as I can see there isn't the possibility to pass extra flags to build command. Am I missing something? There is a better way to achieve that? Any help is really appreciate, I wouldn't maintain one project for each client.