[NoRightNoWrong]

Hi! This is my first post at this community. I was wondering if someone could please help me.

Here's the thing: I've got a Kindle 7 and, unless I convert .mobi files into .azw3 format, I'm unable to switch to my custom fonts. *But* the other day I found not one but two books in .mobi format which allow me to choose those custom fonts, without converting to .azw3 or any other format.

How can this be possible? If there's a way to change fonts while reading a .mobi ebook, which is it? (Is there any app to analyze those files' setup, configuration, metadata, etc.? I'm clueless about what makes those .mobi files different from the other ones I've been handling so far).

Thanks in advance for your advice.