How do I change default theme?

I choose pink theme by mistake, how do I change the default theme?

If you go to preferences -> interface you can change the theme you are using. Is this what you mean?

No, the default site theme that every user sees before they change.

To change the default theme you need to go to admin -> customise -> themes go to pink theme and uncheck the Theme is enabled by default. Then go to the theme you want as a default and check the Theme is enabled by default