Habpanel - day/night switch background by item

How can i switch the background for day and night skin? How can i use a openhab item to switch the background color wich comes from a CCS-file. I will not use for every line in my widget the ng-if statement.
I definied my CSS-file the backgroundcolor as an variable. Os is it possibel to change this varibale from black to white by an itemstate(for example a time item if it is 22 o’clock switch from white to black)