You CAN, however, use addons to change fonts, but only in the sense that any addon which modifies a Blizzard default interface element can also have a custom font chosen. Blizzard's UI uses something like 90% Arial Narrow (which is a SUPER common font elsewhere, and you WILL break other programs if you try to replace it), 9% Fritz Quadrata, and 1% whatever the quest header font is (the original Diablo used that font as well but I forget the name, and Blizzard didn't create it despite its ubiquity in that game). Blizzard doesn't have a proprietary font. This means you'd break other programs trying to reference that font as well, and I'm not even 100% sure that's possible with fonts because their nomenclature data is PART of the file, I think, rather than just a label like with most files. To overwrite the blizzard fonts like that, you'd have to erase it from your computer completely by renaming the font you want to use with the name of the font you want to replace and overwriting it.
You can't really make the default UI use a different font through messing with files because the call system references your Windows font folder.
