Well it isn't that certain computers have certain files, PokeMMO uses them, which means that the computer doesn't read them, PokeMMO does. you download the zip/rar (download the unarchiver if you don't have it already, helps with opening files that macs wouldn't normally be able to) of the gui/theme, then do what i said above put it in, you may have to restart PokeMMO I'm not sure, then you open settings, ingame or at the menu go to interface and click theme, then obviously click the theme option at the top and choose whichever theme you want. (Also because I don't see any guides on here i'm probably going to post this kinda guide as a different thread)