To change your calendar background:

1. Head to the calendar (📅) tab.
2. Tap Settings (⚙️) in the top right hand corner.
3. Under ‘Calendar colors’ at the very top, tap ‘Theme’.
4. Select the background that you’d like your calendar to appear in.

🔎 Note: Use the "Dim in dark mode" toggle depending on how you want the theme to act when you're in dark mode.