Resource substitution property

I could be mistaken, but isn't there a property that can be used to substitute resources for different files? For instance, if I have a resource in my app called main.css then in Properties.dev I could specify 

er.something.something.app.main.css=devmain.css

To substitute the devmain.css in for the main.css.

I've dug around a bit but not having any luck finding it. Maybe it just doesn't exist.

Ramsey
------------------------------------------------------------------------------
See everything from the browser to the database with AppDynamics
Get end-to-end visibility with application monitoring from AppDynamics
Isolate bottlenecks and diagnose root cause in seconds.
Start your free trial of AppDynamics Pro today!
http://pubads.g.doubleclick.net/gampad/clk?id=48808831&iu=/4140/ostg.clktrk