hi! i think this is not supported at the moment, mostly because the xlsx extension just output files with .xlsx after that probably macros format can be added to the xlsx template, but i'm not 100% sure about that because i don't know how excel with macros xml format look like. let's see if we can do some research and see if it is easy to implement it in the current xlsx recipe. if you are willing to experiment another option for you can be to look at the code of xlsx-recipe and try to tweak it to add the support for xlsm files, basically making your own custom recipe based on xlsx recipe code.
Went through the tutorial, but it doesn't cover what's not supported as far as CSS, and doesn't link I to resources needed to learn the default engines or recipies. Flex box is working perfectly with the appropriate resets. Only when I try to use the helper.