@jackie_w,
Thanks for your clarifications on the userStyle.css.
Well, correct me if I'm wrong but there must be one file (or two one for kepub, one for epub) that hosts all supported css/html tags and defines all default settings.
Code:
@import url(file:///mnt/onboard/my.css);
Maybe it will work if you place your css in the fonts directory. This directory path is predefined by the system for locating external fonts.
Code:
@import url(file:///fonts/my.css");