List all fonts used in a powerpoint (pptx) document using with open XML SDK

For XLSX it's easy, you just need to read "styles.xml" part and check the "styleSheet" -> "fonts" element.

For DOCX it's not that easy.
First the document does not have to have "fontTable.xml" part.
Second the "fontTable.xml" can have less or even more fonts then what is actually used in the "document.xml".
So to get the correct information you would need to check all "rPr" -> "rFonts" elements in "document.xml" part, but also in all headers, footers, etc.

For PPTX there is no centralized part which contains the font information. It is scattered everywhere, it can be in "presentation.xml", in master slides, layout slides, regular slides, etc...
Which is why I would recommend you to use an alternative approach, for example check this PPTX example in C# and VB.NET, it shows how you can work with that format and how you can obtain strongly typed slides in C# and VB.NET. What you would need to do is iterate through presentations whole content and search for TextCharacterFormat.Font.