List themes are a way to differentiate between different lists in your account. The options are the same as those available to your whole app themes (found under your app settings), including the option to upload your own custom backgrounds. List themes are available to Plus subscribers and higher.
To set a theme, tap/click on the name of the list. Select "Manage add-ons" and tap/click "add" next to "List Themes".
A button called "List Themes" will appear under "Manage Add-Ons", with which you can personalize your lists.
You can choose between predefined colors and images or upload your own images.
List themes overwrite your selected app theme: If you have set a theme for a list using the add-on, you will see that theme instead of the theme you selected in your app settings.