K5 how to add alternate fonts

In the fonthack k5 thread someone had mananged to have own fonts without replacing system fonts.I created an empty file at the root and put averia fonts directly in the font folder and did ;fc-cache and pressed enter.then I went away for fifteen minutes and when I clicked aa to select font in books the kindle restarts.after I delete the USE_ALT_FONT file it goes back to normal