Currently, sites need to have a colour scheme selected as the dark mode colour scheme - it isn’t built into the colour palette system to have two modes.
We are, however, deep into work that will make this much more intuitive and include native light and dark modes for all themes, with user preference deciding which they will see. We don’t have an ETA for these updates just yet, but we’re very far along with it and hope to have light and dark modes for colour palettes (along with other related improvements) out soon.