A theme can be applied to a design only if it has a source of natural light (example; an external door or a window).
If a design does not have an external door or a window, a theme cannot be applied.
In order for theme to work, please add a window or an external door in your design.
Note: If no doors or windows are added, 2020 Fusion will switch to using the lighting rig.
Related articles
How to Set a Theme in 2020 Fusion
How to Import and Apply a Custom Background in 2020 Fusion