This is a summary of where I think we're at with Kobo GUI system fonts:
Part 1: - "fooling" the Kobo into using different physical font files
A "better
Avenir Next" solves a problem for Cyrillic users but not necessarily, for example, for Vietnamese users, and definitely not for those who require CJK fonts.
If a better
Avenir Next does not solve the problem for your language it's probably not dodgy to say that any font the user has legal access to can be renamed using JSWolf's instructions in post #57.
- Users who like having 2 system fonts (1 serif, 1 sans-serif) can pick 2 "good for my language" fonts and use the renaming trick to Georgia (serif) and Avenir Next (sans-serif) before sideloading to /fonts with the correct filenames. See N.B. Update below.
- Users who only want a single system font, using their "good for my language" font, can do the renaming trick twice with a font editor: once to Georgia and once to Avenir Next before sideloading to /fonts with the correct filenames. See N.B. Update below.
However, using FontForge for the renaming trick, whilst conveniently free, is not necessarily a happy experience for a non-techie. It's not particularly user-friendly or robust [**].
N.B.: *** Update for firmware 4.21.15015+ ***
Once the renaming trick is complete, the info in post #2 will be required to force the Kobo to load all its fonts after a re-boot.
In fw 4.21.15015 @Semwize discovered (
post #91) that it is now possible to completely overwrite the default Georgia and Avenir system GUI font files rather than sideloading copies to /fonts. See
post #96 for some sample code you can use in kobopatch to copy your custom fonts into the main firmware font directory. If you do this it will no longer be necessary to perform the shenanigans in post #2 after every re-boot.
Part 2: - using kobopatch to change Kobo nickel CSS for font-family.
N.B. Unfortunately, in fw 4.32.19501, Kobo made major changes to font handling. A side effect seems to be that any kobopatching of font-family is so limited as to be no longer worthwhile for any practical purposes.
For those who are comfortable using kobopatch but not a font editor ...
If the only part of the Kobo GUI you really need to improve is the My Books book lists then- the fairly generic patch for nickel in post #10 can be used with your sideloaded font(s) of choice.
- Chinese users, and others who use Code2000 as their preferred sideloaded font, can use a similar patch in post #43
There are also patches in the kobopatch standard pack which can customise font-family in other parts of the GUI, e.g.- 'Custom new header/footer font' (not Japanese/Chinese users)
- 'Custom page navigation scrubber' (not Japanese/Chinese users)
[** My personal experience with FontForge, on Windows, is that I just hope that I manage to get what I want done before it crashes in a heap.]