to my shame i did not know how to customize the nmtui default colors (for the sake of my own health and mental stability). after reading a bit, i found out it uses the so-called newt backend with the whiptail app.
long story short... turns out it reads some of the env variables for that backend to setup the colors (if you're interested, here's where this happens in the code).
so you can simply set these env variables before calling nmtui. here's a combination i came up with:
NEWT_COLORS='root=black,black;window=black,black;border=white,black;listbox=white,black;label=blue,black;checkbox=red,black;title=green,black;button=white,red;actsellistbox=white,red;actlistbox=white,gray;compactbutton=white,gray;actcheckbox=white,blue;entry=lightgray,black;textbox=blue,black' nmtui
enjoy and keep your eyes healthy!
Probably a Scandinavian country if I had to guess.