As far as I know, only the Simple theme is installed by default. If you want more themes: just install them!
To install a new theme you only have to follow these steps:
1. Download the theme package that you wish to add to Publii; in this example we’ll download the starter.zip package downloaded from the marketplace.
2. Once the download is complete, open Publii and navigate to the Themes by clicking on the three dots in the top-right of the Publii interface to open the submenu, then clicking the Themes option.
3. Drag and drop the downloaded .zip package onto the themes section; Publii will automatically unpack and add the downloaded theme to its library.
4. At this point you can follow the standard steps to install the new theme on your website; the Theme drop-down list will automatically include the newly-added theme in the list of themes available to install and use.
The following themes are free:
• Starter (https://github.com/GetPublii/theme-Starter)
• Technews (https://marketplace.getpublii.com/themes/technews/)
• QF Fashion (https://marketplace.getpublii.com/themes/qf-fashion/)
• Portfolio (https://marketplace.getpublii.com/themes/portfolio/)
• Mercury (https://marketplace.getpublii.com/themes/mercury/)
• Tattoo (https://marketplace.getpublii.com/themes/tattoo/)
• Taste (https://marketplace.getpublii.com/themes/taste/)
• Square (https://marketplace.getpublii.com/themes/square/)
So in total you have nine free themes!! That’s a lot!