See this: A more compact way to do "conditional" label/icon/color/whatever in MainUI page - #3 by jimtng
Instead of items.YourItemName.numericState
use: #'YourItemName'
. This is documented here: Widget Expressions & Variables | openHAB
background: =#'Heizung_Wohnzimmer_Aktuelle_Temperatur' > 21 ? 'red' : #'Heizung_Wohnzimmer_Aktuelle_Temperatur' > 19 ? 'orange' : 'blue'