I try to modify my opencart a bit. For my shop i need 2 differents themes for the bills. I thought this should be quite easy. I just copied the Button "print bill" (I use german language pack, so i am not sure if its exactly this words in english version). My hope was now that i could just change somewhere the path to the tpl-file and that I will have 2 themes. Somehow this doesnt work

My level of understanding PHP is very low, but I tried to find $invoice in the files. But this variable is nowhere... I can only find $invoice_no and this totally confuses me.
So my question is, if my way of trying to get two different themes for the bills is just stupid or what do I have to do? Or is there even a much simpler way of solving this problem?
Thanks for your help!