This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
Include Static Template¶
Create a new extension template (TypoScript) or edit an existing one and include the static template of wt_cart. With wt_cart 1.2 comes an additional static template for the CSS default styles ('Add default CSS (wt_cart)'). So if you want to include some default CSS styles you can do it that way.