formatting like in Seraphinite Post .DOCX Source

I’m afraid I’m not familiar with Seraphinite Post .DOCX Source. It’s worth noting that Mammoth is designed to work with documents that have been semantically marked up using styles, rather than attempting to copy the exact formatting of the original document. Other than basic formatting such as bold and italics, ad-hoc formatting, such as changing fonts, isn’t going to work.