Mobile Background

Hey,
i want to modify my Orizion theme that, wehen watched from a mobile device, the background changes to a repeating tile i created (27x63px). How can this be achived?

i found out that the theme goes to mobile mode with a certain width but i havent found the needed css file.

can anyone provide me the file name and, if possible, the code to define the background tile like
url(xx/xx.jpg)

thanks anyway