Modularity Lite – Custom background on category page

I am trying to put in a custom background and have tried a few things with this template Modularity Lite and would like to know how to go about adding it .. I have 3 categories so need to be able to do for all 3 then more if needed..

I have also tried a few of the things found on the forum here and nothing worked for the custom background though I did get the custom header to work with the plugin Category Templates Two. the other question to this is there a plug-in I can use instead of working with the code..????

Hope I am not too confusing with this..