This is all I needed to do so: https://www.mobileread.com/forums/showthread.php?t=314220
Run the script from the second post, then eject the Kobo and let it install. Afterwards you can open it from the new NickelMenu button at the bottom right. My Kobo just stays in KOReader mode all the time.
Wow I feel dumb for not thinking of that. In my defense I like the text as #FFF on gray. KOReader’s arbitrary CSS snippets and style tweaks are really neat.