The collection themes add-on allows you to set a particular theme or background for a single collection. Every member of the collection will see the chosen collection background, as it overrides the profile background.
Enable collection themes
To enable collection themes, click on the collection name, then on "Manage Add-Ons", then click "Add" next to "Collection Themes".
A pop-up window will appear showing all the available themes and backgrounds. To set one as the collection theme, click on it and hit "Done".
If the collection is part of a Plus workspace or higher, you can add your own custom background image and theme.
Learn more about customizing backgrounds.