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There is no way to set everything back to default, because no such thing as the default settings exists.
What I and some others here do, is create a test book for messing with fonts (I have a 5-page epub filled with nonsense text). Then I looked at my current settings in the Kobo configuration file (by connecting my Kobo to my computer) and saved them in a text file on the computer, so I'd know what they are. After that, I used the Kobo Utilities plugin to set the reading font for all the books on my Kobo, using those saved settings (it also sets the margins and line-height, not only the font).
Now I can try out fonts in the test book, because the settings for all the other books are already saved and won't change.
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