allow dark_mode for (all) maps to be set in theme

[Mariusthvdb]

currently we can set

dark_mode: true/false

on a single type: map card. This is however a fixed setting, and can not be influenced by theme color settings. Also, this is not what

modes:
  dark:

is used for in themes, which makes the HA theme follow the device settings for dark/light.

it would therefor be really welcome to be able to set a single theme variable for dark_mode: in the themes, so we can have all maps show dark:

• type: map cards
• sidebar Map
• more-info on device_trackers (lat/long entities)

hope you will consider. thanks!