GDOv7 comes with it's own template language and syntax.
Just kidding!
GDT_Template :) uses plain php as templates.
GDT_Template needs a modulename and a filename, optional parameters and.... your theme chain.
A theme is an alternative template files for a modulename+filename combination.
the GDO_THEMES /protected/config.php variable defines your theme chain, like: "mytheme,bootstrap4,classic,default"
@TODO add $path to GDT_Template for an static filepath template.