buildThemes: sass.render() is deprecated

[tobiu]

the new methods are compile() and for our use case compileString(): https://sass-lang.com/documentation/js-api/modules#compileString

i did a quick test and unfortunately the API and way of using the new method differ. in detail: passing the data as the 1st param is an easy change, but afterwards the internal import URLs inside .scss files break. me might need to add a custom importFn to handle our use case.

help on this one is appreciated.