If your host system is set to light theme, then in Shotwell you can toggle the theme as expected:
Edit -> Preferences -> Use dark theme (toggle). When you toggle it to enable dark theme, the app becomes dark and when you toggle it again the app becomes light.
However, if your host system is set to dark theme, then toggling the theme within the shotwell app does nothing.
If your host system is set to light theme, then in Shotwell you can toggle the theme as expected: Edit -> Preferences -> Use dark theme (toggle). When you toggle it to enable dark theme, the app becomes dark and when you toggle it again the app becomes light.
However, if your host system is set to dark theme, then toggling the theme within the shotwell app does nothing.