About Customizing Templates

<ons-list-item> does not have a template.When compiled, classes such as .list__item and .ons-list-item-inner are simply added.

You want to customize it, but you can use the modifier attribute if you just want to look good.
http://ja.onsen.io/reference/ons-list-item.html#modifiers

If you want an elaborate HTML structure, you don't need to use <ons-list> in the first place.