... follow-up, I see I took so long to reply that others have chipped in
I can quite understand that you may not be comfortable putting customised firmware on your reader e.g. 'Russifying' or PRSPlus, but just for completeness I thought I'd add a bit more info about them as regards epub font customisation.
Both of them allow you to set up a specially named .css file on your reader which contains all the @font-face statements you've decided you like. This has the benefit that you may not need to edit the .css file inside each epub at all.
Where they are really beneficial is:
- when you're reading an epub with DRM. You can't edit the css inside a DRM'd epub but it CAN use the fonts specified in the special css file on the reader.
- when you have 2 different types of reader and you want to read the same epub on both. For instance, I have Sony and PocketBook. The @font-face statements are different for both readers. Not because the font files are different, but because the location they need to be stored in is different (the src: url(res:...) bit). Both readers allow use of the special css file on the reader itself (the Sony via 'Russify' or PRSPlus, the PocketBook using a similar "hack") so both on-reader css files can be different and the same epub can be read on each without editing.