It is currently set to "browser default" meaning if you set your browser to prefer dark mode it will switch to it automatically.
If we would set it to dark mode it would default to that even for those that explicitly set their browser preference to light, which is IMHO not so great.
That said, now that Photon got theming support and some performance optimisations it is not unlikely that we will switch to it as the main UI in the coming weeks. It should be seamless as it can convert the lemmy-ui login cookies as well now.