Describe the feature:

Currently, an advanced setting theme:darkMode controls whether you see Kibana in light or dark mode.

Instead of having a simple toggle (dark mode on/off), it'd really be nice if there were three options: Light mode, dark mode, or browser setting (similar to how we do timezones). It'd be especially nice for those Mac users who have the preference set to "auto" so it changes based on the time of day automatically :)

Most modern browsers support inferring the user's preference via window.matchMedia('(prefers-color-scheme: dark)') (or light).