Check this reply: https://github.com/pacocoursey/next-themes/issues/15#issuecomment-720168813

The defaultTheme is only used for new visitors to your site. As soon as they visit, the defaultTheme will be stored in localStorage, so if you change your ThemeProvider value of defaultTheme it won't matter, since localStorage will take priority after the first visit.

Try opening your page in incognito and check if default theme is dark.