@Swen well, you cant.
For the third time, it is to test if your rule works.
Because if the variable updates, and comfy is not highlighted, it is not possible.
Eco and comfy are hard coded and i think they cant be used in a rule to highlight.
Let say you set eco on 15 and comfy on 21 in your config.
You have a rule: set temp to 21. Then comfy should be highlighted. If you set it to 15, eco should be highlighted.
I dont think you can use it as following;
WHEN eco is pressed THEN set temp 15. Otherwise setting it in your config would be useless.