As you can see in Youtube, there you can find a switch to change between “normal mode theme” with bright colors and a “night mode theme” with black and gray. GNOME and its default theme Adwaita has a normal theme and dark theme. This kind of thing help people who works with the computer in day and night by allowing to adjusts the mode better suits them.
My idea it’s to add a “night/dark mode” that unlike the examples above, works automatically, changing from normal mode to dark mode when certain time has passed, using system time or hardware clock as reference (e.g. it automatically enables itself at 10PM and disables at 8AM).
In order to maintain consistency and a clear look, all the themes should have a normal/daylight mode and a dark/night mode. If there is a theme doesn’t have an alternative mode then it uses the same look all the day. If there is a theme that’s dark by default, it’s up to the developer to find a way to make it a night mode or not.
Another thing would be also to automatically adjust color temperature from colder to warmer tones depending on the time of the day, that could be also another option, or both at the same time.