OpenCart comes up with a default theme, and also there are a lot of free themes available for OpenCart. So, why do we need to customize our theme? We can use the ones that are freely available. But, the freely available themes do not match our designing style. Often, they lack modern designing techniques and layouts. Some people prefer to show a single-column page, some double-column, and some a triple-column layout. So, to attract users to our site, we need our design to be attractive and modern.
Source: Hasan Tahsin (2011), OpenCart 1.4 Template Design Cookbook, Packt Publishing.