In Writer, it is possible to quickly set the default font in Tools/Options/LibreOffice Writer. This functionality should be similarly available in LibreOffice Calc, rather than having to use the far more obscure process of having to modify the style without a clear way of saving the modified default style as a permanent change, then saving it as a new template, and finally having to save the template as the default template.
Why don't you request that to LibreOffice?
*** This bug has been marked as a duplicate of bug 33743 ***