To be super clear: there's two toggles:
The first one (Theme) allows you to switch between the default theme and the customized Material Dark theme.
Also important, though, is the second switch here. Color Scheme / Regular is used if your web browser is in "light mode" or doesn't indicate a preference, and Color Scheme / Dark is used if your browser indicates a preference for "dark mode".
If you don't want to change your device (or have a browser that doesn't report a preference), you can change the regular color scheme to dark.
Yes, it is a bit more complicated than it could be.