What did you modify with regard to Charis SIL, and what software are you using? If I'm going to modify something myself (mainly, add some weight), it'd probably be Trebuchet or DejaVu Serif.
A font size setting using a number, and uploadable fonts should be default on any ereader. Many older ones actually had this (or at least partly), such as the mentiond Cybook Gen3.
I have a feeling that I'm just stuck: gotten used to Caecilia 4 on my Touch, and now everything else will look "wrong" in the end. Just like the PW light: it's stuck on setting 13 when at home, in my usual two reading places. 12 is already too dark, 14 too bright. If I'm in a different spot with more or less light, I'll modify the brightness in such a way that the Kindle looks the same (with regard to overall brightness) as if it was at 13 in my usual spot.
Stuck I say, stuck!
