If you are on Linux you should be able to create a shell script that change your system color to dark or light depending on the time. On gnome there may even be some extension that do that already.
If you use Windows there may also be options to do the same with task scheduler.
They turn white for me (or green for sticky posts). However, there may be some combination of dark mode and theme setting that I've overlooked. The #app id should have the attribute data-bs-theme="dark" if you use darkly. If your entire system use dark mode it should also work by default if your system tells your browser that you prefer dark themes. (But data-bs-theme will still be set to light unless one go to the settings and change the theme to darkly.)
Rather than manually adding communities, could it be made to fetch active/hot communities from for example: https://lemmy.world/api/v3/community/list?sort=Active ?
There are plenty of userstyles and userscripts inspired by old reddit if you don't want to use mlmym. See !plugins@sh.itjust.works.