Control light/dark scheme preference

[annybs]

At the moment, the dark colour scheme is the default and the light one is only activated when the system specifies a preference for a light scheme.

This should remain the standard behaviour but an option should be added to account settings enabling the user to set their preference explicitly if they so choose, i.e. dark, light, or follow system.

This could be stored with the account or just in localStorage - TBD.