There is only one default theme and it has no CSS.
The theme used is hardcoded. I will add a feature in the future allowing you change this from the dashboard.
To create your own theme, add a directory titled the name of the theme under Resources/Views/Themes/. Then create index.leaf, page.leaf, and post.leaf. Link to any custom CSS or JS you add to the Public folder.
Database
Mist uses PostgreSQL.
Admin Panel
The AdminPanel can be accessed by going to $PORT/admin/login
If you seed the data to the DB, the admin credentials are admin@admin.com and admin.