When I first got the Kobo Glo I tried the customised css technique which I know works on a Sony (unhacked, unrooted) device.
Based on the Kobo filenaming convention used for book cover images it looked as if the Kobo sees the int.memory root as file:///mnt/onboard (or maybe res:///mnt/onboard ???) in which case I thought adding
as the first line in each epub's internal css file might be a way to add my personal css (as contained in my.css file in Kobo root) to the existing css. Unfortunately I didn't have any success
The only way I've found, so far, to semi-automate what I wanted to do was to write homebrew code which cycled through
- explode epub
- add list of my css rules to css file
- rebuild epub
for each selected epub on the Kobo disk.
If the Kobo was my only ereader it's possible that I could have achieved the same result with a Calibre epub conversion using the ExtraCSS feature. However as my epubs need to be read by Kobo, Sony, PocketBook and Android, this wasn't workable for me.
If you have any bright ideas I will certainly be interested to hear them.