During 1st -time installation, you’ll have a choice to select either light or dark theme.
I chose the light one.
I thought, at least I’ll be able to switch between light and dark at any time, but under settings, I see that I’ll have to install.
You can set the ‘default dark mode color scheme’ in admin/site_settings/ under basic setup. That sets the colour scheme if the browser is set to dark mode.
To install the dark theme as well, just click install in admin/customize/themes and select dark from the list of popular ones. To make that default once installed, select it and check the box ‘Theme is enabled by default’.