After learning something new ... It is actually quite easy
Define the style(s) in your node.html
<style>
.homie-ready {
background-color: #1f7a1f;
}
.homie-lost {
background-color: #AD1625;
}
.homie-sleeping {
background-color: #4682B4;
}
.homie-init {
background-color: #66CDAA;
}
.homie-disconnected {
background-color: #696969;
}
.homie-alert {
background-color: #FFFF00;
}
.homie-undefined {
background-color: #ffcc00;
}
</style>
class: `homie-${device.$state}`