How to change gmail dark mode

You can choose between dark, light, or your device’s default theme:
On your Android phone or tablet, open the Gmail app .
At the top left, tap Menu .
Tap Settings General settings.
Tap Theme.
Choose Light, Dark, or System default.
Reff

@mudassir-afzal
We are talking about gmail not browser

You can change the theme of your browser in General Setting

@zaasmi well answered

gmail dark mode is not available for everyone.