Adapt a Theme built for another e-commerce platform?
We have a completely custom e-commerce platform built using Ruby. The site needs a better theme than what I have created for it - better CSS, a nicer layout, and needs to be responsive. I've used several shopping cart and CMS systems that have tons of themes you can buy that are easily plugged in. I was wondering how difficult it might be to go out and choose a theme from a site like ThemeForest, one that was created for one of the popular shopping cart systems (like Magento or OpenCart), and customize it to work with our site. I know there will likely be quite a bit of work trying to adapt it, but at this point it might be worth the effort since we have 4 sites that need to be converted into our system this year - I don't think creating new themes from scratch is something we can do ourselves. I can adapt the themes in-house, as where we'd probably have to contract a design company to create designs and code them from scratch, which would be quite expensive for all the sites.
Any advice on adapting a theme made for a different system? Do any lend themselves to being adapted more than others? For instance, would a Magento theme be easier to adapt than say, one for OpenCart?
Thanks in advance!