First of all, visit the customization section in the official store here. You'll find for example a pride theme, or a tool to randomly change the background with unsplash.
Be careful, some of them might be very old, probably too old for your instance, and I haven't tested them all.
- Breeze Dark theme, link to the Nextcloud appstore here
- Dark theme
- Elemental theme
- Material Dark theme
- Material Light theme
- Phex theme
- Pride theme, link to the nextcloud appstore here
- UMECloud theme
You can find a bash script to change the icon here.