To change your calendar background:

1. Head to the Calendar tab (📅)
2. Tap Settings (⚙️) in the top right hand corner
3. Within the ‘Colors’ section, select ‘Calendar 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.