Tuuli comes with several color schemes by default and you can select which one is the default from the Admin.
Here are the color schemes you can choose from:
- System (dark or light depending on the user preference)
- Light
- Dark
- Midnight
- Ivory
- Skyblue
To select one the default, simply go to Settings > Design > Site-Wide:

You can also customize the existing color schemes, for more info check out the documentation.