Do you have a Zen Cart website? If so, yes, the only way to alter the look (without coding it yourself) is to install a Zen Cart template.

If you have an existing HTML website, you would replace it with a Zen Cart complete installation and a Zen Cart template of your choice. Don't blindly install Zen Cart on top of your existing website or you will have huge headaches; rather, read up on the process (in the FAQs here) and install ZC in a testing location, get it working the way you want and move it to replace the existing site.