Yup, right now it's added as the last stylesheet (since I assumed (maybe wrongly) that was roughly equivalent to what happens on kobo's files where that bit of css is inlined at the bottom of the head tag).
IIRC, kobo_extra is a bit more complex, and basically rewrites everything and/or appends itself at the end of *every* stylesheets.
On a sidenote, realized why I was confused by the !important thingy in my quick tests: I'd missed a *third* div wrapping the contents of the body in my test book, which of course threw me since I thought it wasn't there ^^ (serves me right for trying to gain a bit of time by reusing an existing thing without checking it more carefully). Thanks for the detailed explanations & references
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