For those interesting in epub Fonts usage, the public Fonts directory of Cybook Opus is in fact /mnt/fat/Fonts/. The internal Fonts folder is "/mnt/app/res/fonts". You can find them in the system/.fonts-cache-1 text file.
So after a lot of reading and testing I successfully used the Liberation fonts (available in Calibre) in my epub files. Most importantly, I was even able to embed these fonts in the epub files so ADE could show Cyrillic letters instead of ???. You can read this thread if you need a portable epub file with embedded fonts:
https://www.mobileread.com/forums/showthread.php?t=61587
However, if you want to use the Liberation fonts only on your device, first copy all fonts from "C:\Program Files\Calibre2\resources\fonts\liberation" to your Cybook Opus Fonts folder. Then edit the stylesheet.css file (or enter the following text at Calibre -> Look & Feel -> External CSS field):
----------------
stylesheet.css - insert the following text after the first line
----------------
@font-face {
font-style: italic;
font-family: 'Liberation', serif, sans-serif;
font-weight: normal;
src: url(res:///mnt/fat/Fonts/LiberationSerif-Italic.ttf);
}
@font-face {
font-style: normal;
font-family: 'Liberation', serif, sans-serif;
font-weight: normal;
src: url(res:///mnt/fat/Fonts/LiberationSerif-Regular.ttf);
}
@font-face {
font-style: italic;
font-family: 'Liberation', serif, sans-serif;
font-weight: bold;
src: url(res:///mnt/fat/Fonts/LiberationSerif-BoldItalic.ttf);
}
@font-face {
font-style: normal;
font-family: 'Liberation', serif, sans-serif;
font-weight: bold;
src: url(res:///mnt/fat/Fonts/LiberationSerif-Bold.ttf);
}