Opencart theme development, in it’s current state is flawed in my opinion. Opencart is a great platform for any type of ecommerce store, having a plethora of features for users and a low entry point for developers. As a result, Opencart has plenty of themes available for download, just take a look on Themeforest. Occassionally, to test out various things out I’ll install such themes locally. More often than not the theme will break the store completely, or render parts of the site completely broken.
9 times out of 10 the issue will be down to the theme developer not sticking to basic Opencart theming guides – they’re very basic to say the least. The main point to note is that Opencart, like other ecommerce platforms, uses a fallback technique on the “default” theme. So, for example, if Opencart doesn’t find a template for a category listing page, it will fallback to the template from the default theme. If the latter is not adhered to, and it isn’t – look at some of the so called “premium” themes out there, it makes upgrading very, very painful. There are theme developers out there that are copying over a files for the sake of it, when no changes are made. If your theme literally only has changed the color/layout of the footer, apart from the default stylesheet folder, image folder and header template you only need the footer.tpl in your theme folder.