The latest versions of the Fonts Hack do support tweaking the fonts in KF8 files
.
But, before you embark on that path, you might want to check the styling of your book, there's a 'feature' (hint: it worked fine on older KF8 renderer (Touch)) where some font* styling directives are followed, even when they're stupid, preventing the Kindle from switching fonts, and falling back to the default system sans/sans-serif/mono families. (A quick search here & in the mobi forum might reveal more details about that, I never bothered to try to fix it myself, but I do have several books displaying this behavior
).