personally, I would just compare the Style Coding of a v.1.5.6.x responsive Theme Checkout File with the default OC v.1.5.6.x Source, to find out, what has to be changed, to PLACE the Stuff by CSS, and not longer (usually) by HTML Table-Arrangements, to make it responsive. But I am not familiar with Journal Themes, so I don't know, wich
Road they follow to get the stuff responsive, but I assume, it only takes an assortment of Style Commands, to also make a 'default' Checkout File responsive.
The AGNES 1.5.6.x Theme is responsive, and free, along with two hand full of others. It could serve as reference, in case, you have some knowledge on Stylesheet Stuff and PHP.
Good Luck, just figured...
Ernie