Hi there @urobepi -
You cannot save changes made to the predefined Default theme. So probably you are using the Default theme and that is why the option is greyed out.
The workaround is to load this theme called White, which was generously shared by another user. It is identical to the Default theme, but is a custom theme so it can be saved.
Unfortunately, when you load White you’ll lose the changes you’ve made to Default, so you might want to make note of them somewhere first, so you can recreate them later with White.
Best,
Jim