Tzeentch

Veronica Explains comes to mind: https://www.youtube.com/channel/UCMiyV_Ib77XLpzHPQH_q0qQ

She is also on Peertube: https://tinkerbetter.tube/c/veronicaexplains/videos

This sort of exists https://reactos.org/

Key word on sort of, it’s quite rough in many areas

Fennec enables access to about:config, so making all of arkenfox’s changes manually(by viewing its user.js and toggling it in about:config accordingly) would also be an option, none of arken’s toggles are desktop only as far as I know

The themes section on the desktop integration doc here might be of use: https://docs.flatpak.org/en/latest/desktop-integration.html

This may be useful for further elaboration: https://www.reddit.com/r/flatpak/comments/y9jmqj/the_general_flatpak_qt_and_gtk_theming_guide/

Going off of these guides my best guess would be that you don’t yet have kvantum or some other theme runtime installed within flatpak

I’ll also mention Stylepack exists for automating and streamlining theme installation, it’s only for GTK currently however but maybe will be of use in future: https://github.com/refi64/stylepak