Custom card theme config

[rigrig]

Is your feature request related to a problem? Please describe.

I've got a "Battery Warning" card for my front page, and I'd like to apply a custom theme to it, so it stands out when there is a problem.

Describe the solution you'd like

A theme: <string> config for the card: Override the used theme for this card with any loaded theme, same as for built-in cards.

Additional context

My current setup, using custom:auto-entities with an entities card and theme: uglychristmas:

card YAML

type: custom:auto-entities
card:
  type: entities
  title: Battery Warning
  secondary_info: '{attributes.battery_type_and_quantity}'
  theme: uglychristmas
filter:
  include:
    - entity_id: '*_battery'
  exclude:
    - state: '> 25'
    - state: unavailable

Using battery-state-card would mean no need for the auto-entities wrapper, and being able to show the battery type attribute, so I know what to replace.