I see some people have difficulties with this hack over-riding a font that's embedded in the epub. This can easily be prevented (in the vast majority of cases) by restricting the font-family definition to the html tag only, thus:
Code:
html { font-family: serif; }
[do not include any font definitions for the body or p tags]
This way the top-level html tag will specify your replacement font. But, most epubs with embedded fonts put the font-family definition in the body or paragraph tags. These will now not be affected by the global user css file and will take precedence over the font-family attribute in the html tag.
I use this hack with PRS+, and this saves having to disable the user css for epubs with embedded fonts.