Provide a way to choose the theme using the device's default configuration

[tbouffard]

We have recently added the support of the device's default configuration in #226.
But once we toggle the theme selector, there is no way (except if we remove the entry in the local storage which is far from being user friendly 😸 ).
Currently, we have a toggle button that only to select 2 state: dark or light.

To support a 3rd state (device default), we have to change the UI design of the selector.

Existing solution

web.dev

We could provide something like in the https://web.dev/?hl=en which provide tooltip on the selected theme icon as well.

web.dev_theme_selector.mp4

tailwindcss

https://tailwindcss.com/docs/dark-mode

tailwind-css.mp4