You already have a light and dark mode, but they are only to be switched manually.
There is a quite new CSS media query feature called prefers-color-scheme. It allows you to detect whether the system/user wants a dark website, so you can adjust your CSS.
Currently it is only supported by Safari and Firefox 67 or higher.
Anyway, it would be great, if you could make use of this. IMHO just set the default theme to the one the system wants, if the user did not set it manually…
BTW: I also have a Firefox add-on called “Dark Mode Website Switcher” (source code) that you can use to toggle this setting in Firefox directly – without needing to change the system option. Maybe that could be helpful.