gnome-shell-theme-e-ink
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Buttons on the shutdown dialog are black
…and no text can be read 
Not black in my environment (GNOME 42 on Fedora 36 + v2.1.0 of this theme).

Could you tell me your GNOME and this theme versions?
gnome 42.4 theme 2.1.0 fedora 36 as well
Sorry, on another user on the same machine with seemingly same settings regarding themes I see all OK:

I confirmed this by tunning E-Ink display parameter to more dark.
Dialog buttons are relatively darker than other components so it seems better to change to more bright.
It seems to be solved on v46.1.0 by rebasing to the default high contrast theme.