I’d like to adapt the temperature icon so that it has an arrow pointing up or down depending on whether the current-previous value is positive or negative. How would one do this?
Also, since the icon will have to be something I design myself, how do I point to it? Currently I’ve only used mdi icons so I’m not sure. Do I put the icon files somewhere in the HA directory?
Use the Trend Binary Sensor to determine if the value is increasing or decreasing: https://www.home-assistant.io/components/trend/
Create a template sensor based on your temperature sensor that changes icon based on your binary sensor, example here: Binary sensor state text
You can use any of the up and down mdi icons or use your own png files, example here: Binary sensor state text