[NiLuJe]

@ATimson: Technically, you can with the Fonts Hack, although, if it's falling back to Helvetica or Futura, it is strongly discouraged to override this family (sans), because it's used basically everywhere in the UI, and there's a very high chance that bad things *will* happen if you don't take precautions (like never rebooting w/ a sans override, never switching to USBMS w/ a sans override, preferably doing it with an FT override active, and w/ a shell open in case things go boom, so, not terribly safe or practical).

A much safer solution is to fix the CSS of the affected books w/ Calibre.