To apply themes you must first purchase them. Sofa does come with three free themes, which can be found in "My Themes".

From Themes Section

Navigate to Themes

Navigate to My Themes in the top right

Tap the "Apply" button. This will present options for how you would like to apply the theme

1. "Light Mode" applies this theme for when your device is using the system light mode settings.
2. "Dark Mode" applies this theme for when your device is using the system dark mode settings.
3. "Both Light & Dark Mode" applies this theme for all system appearance settings.

From Settings

Navigate to Settings in the top right