thanks for the suggestion, but at the moment that's not possible
because Bear uses many fonts (titles, styles, code, etc...). The
user would have to pick more than one custom font and it will
probably become a mess.
We're committed to keep adding new fonts, if you have some
suggestion I'll be happy to note those down!
Not sure to understand what is the issue when you see that other editors (almost all of them) can use custom locally installed fonts... It's maybe more a matter of technical debt, but normally, if a user chooses a computer installed font, the editor must be smart enough to pick italic and bold when and where it's needed inside the edition section. And there is no need to apply the chosen font to all the interface, it's mainly a feature dedicated to the editing experience.