Dark mode should be revertable to un-set state

[davenquinn]

Right now, the dark mode provider defaults to the system-configured color scheme, and uses local storage to store the configured state. Once you have configured the dark mode by clicking the button, it is in a "user-controlled" state that can't be reverted to the unaltered system-defined state. We should allow reversion of the dark mode config back to the undefined state, either by having a popover on the button, allowing a shift-click interaction, a long press, or some other means of signaling to unset the state.

[davenquinn]

We should also allow configuring the dark mode provider NOT to use local storage if desired