Is your feature request related to a problem? Please describe.

Many sites on the internet support dark mode, either as a default, compatible with the browser, or just manually set by the user. Since this is mainly a CSS change and redux/react and plotly both support it, I believe this should not be too difficult to implement.

Describe the solution you'd like

There should be a setting, similar to the current language setting, that allows the user to turn on dark mode. Potentially, there can also be an admin preference that allows dark mode to be on by default.
A quick search turned up a number of tutorials on how to add dark mode using redux/react, such as:
https://medium.com/how-to-react/create-dark-mode-in-react-js-using-redux-hooks-9b4155968050

Describe alternatives you've considered

Continue with a single theme, which is already implemented and working fine.

Additional context

This is an issue I may work on in the near future, but I may not have the time; so it could be a good first issue for someone else since I believe it will not be very complicated to implement.