It doesn't need any CSS since the Nginx error pages use the browser's default styling. The meta tag just tells the browser to use its default light or dark mode styling depending on the user's preference.
A standard behaviour on all sites that properly implement dark mode is that it adheres to the system-wide dark mode setting by default. If the user doesn't want dark mode, they'd turn it off system-wide. The site can let the user opt in to dark mode just for that site, but the default is based on the system setting.