@Sraqa In Opera One, you can find it by typing "dark" into the search field in Settings - you'll find it under Themes as "Force dark theme on websites".
Night mode actually involves reducing the blueness of colors (on devices that have such a setting), Opera does not have a comparable function. Dark mode involves changing the background to a dark color (and making text light so that it is readable against that background) - that is what @leocg is asking.