On a new install, what you get right now is:
- a default theme
- various default color schemes:
So in my understanding, the default dark theme is the default theme & the default dark color scheme.
If you did adjustments already you might need to check the value of default dark mode color scheme id in your general admin settings:

Another small quirk to note is that there’s no default light color scheme listed on the backend. When you pick “+ New” to create a new color palette, the default light scheme is used as a basis though. So if you want to see the color values of the default light scheme on the backend you’d need to add it manually.

