I'm from Wiki staff and I saw you posted on Portable Infoboxes thread that: "mobiles are not ignoring the CSS of custom themes". It's very strange and shouldn't happen. Can you provide the links to broken articles please? :) We'll appreciate your help.
The Champion Infobox is actually two infoboxes: each has a custom width, margin and various other display properties. These styles are affecting mobile displays - at least on my iPhone 6Plus, my mum's iPhone 4 and Chrome's emulator for various browsers.
This is the custom styling I'm using in my User:Emptylord/common.css.