add alert-color variable for applicable device_classes

[Mariusthvdb]

in https://github.com/home-assistant/frontend/blob/dev/src/resources/styles.t we have paper-item-icon-color and paper-item-active-icon-color, so we can use state_color: true and have that applied via our themes. This is default for 'off/on' states.

it would however be even better if we could make several 'alerts' standout, (device_class: problem etc) with an alert_color set in a theme.
As automation/boolean and regular binary now can use state_color: true, an alert_color: true could be set for the 'problem' binaries. Several device_classes could benefit from that.

Please consider adding that to the core theme scheme?