If you downloaded the theme, and want rid of it, just uninstall it.
If you want to keep it, but just want it off to use the 'nightlight', then just force stop it.
Settings
Apps
(name of theme app you downloaded)
select it
A screen comes up with these options.
Perhaps in your app drawer the app will also appear, and maybe by selecting that icon a screen with on/off options or other choices may appear.
There might even be an option available with your quick settings, where an icon can control the on/off of the theme.
Without knowing the name of the app and where you got it, it is hard to guess what options you have available.
