Build OpenCart Theme

If you want to modify something about default theme, the best way is to copy one for yours then modify it.

Here is a tutorial of Build Opencart Theme.

One thing you need to modify by yourself:
Find "/catalog/view/theme/yourtheme/template/common/header.tpl"
On line 19:
<link rel="stylesheet" type="text/css" href="catalog/view/theme/yourtheme/stylesheet/stylesheet.css" />