For the record, I found the “state_icons” to not work in my lovelace UI. But the following worked well:
- type: custom:button-card
template: bootstrap_button
class: col-xl-2 col-lg-2 col-md-3 col-sm-3 col-4
entity: group.opening
state:
- value: 'on'
color: gold
icon: "mdi:door-open"
- value: 'off'
color: var(--text-color)
icon: "mdi:door"