That problem has been affecting some people and different devices. In some cases the dark mode comes back on its own and in others you need to mess with the Gmail app. Try this, go to settings - apps - Gmail - force stop - clear cache and restart the phone.
Yup, it's a funny bug. For many people it happened the same, now it's gone, now it's back. Nothing official from Google yet.Well I opened the app this afternoon and surprise, the dark theme is back lol.