the card_mod is for the dark_sky custom card (note the syntax has changed a bit, add card_mod: :
type: custom:dark-sky-weather-card
card_mod:
style: |
ha-card {
background: url("/local/images/weather-background-{{states('sun.sun')}}.png");
background-size: 100% 400px;
}
and also works on a regular entities card:
- type: entities
card_mod:
style: |
ha-card {
background: url("/local/images/weather-background-{{states('sun.sun')}}.png");
background-size: 100% 400px;
}
entities:
- entity: input_number.test_level_1
name: Level
- entity: sensor.test_sensor_battery
name: Level
- type: section
- entity: input_number.battery_level_critical
name: Critical
- entity: input_number.battery_level_warning
name: Warning
state_color: true
sorry you asked for button-card: wrong thread, go here 
and button-card uses different, Javascript, templates, not Jinja.