"Dark mode" is feature that some operating systems, apps and platforms offer... The open source platform that KU Buckets is built on may or may not have that style as an option. The most likely place such a feature would be available is in the settings area (that's where your profile pic is, in the upper right corner).
If you go there, you can see there is a feature called "skins" that seems to just change the font and nothing more. It's possible that the platform allows more skins modules to be added and maybe there is a skin that changes the style to dark mode. Or perhaps there is an option in the tool for dark mode that just needs to be turned on in order for the option to be available to users.
Either way, this is something the site admin ( @approxinfinity ) would have investigate. If the platform doesn't have the option, and there isn't a "skin" solution, there really isn't an easy way for users to achieve dark mode.