Hey Josue,
Please create the new file under the internet service that you have created. Not under ITSMOBILE service. There mist be a default mobile.css under your service too.
Also, Please let me know the name of the new file that you have created- mobile_e.css or mobile_new.css. Accordingly, you will have to modify the value of ~ITSMOBILECSSINCLUDE in config.
Let me know.