The Html Report Tools are built on the DotLiquid library. This means that the syntax used to create templates is the liquid syntax. This must be used in order to create your own templates and fill them with data. Below are the links to the documentation, both from DotLiquid[1] and from the original Liquid Library from Shopify[2]. The latter has a more comprehensive list of the usable syntax and appears to match the DotLiquid syntax.