There may be a way with CSS, but an alternative would be a rule that populates a variable that you then display on the dashboard.
If you want to go down the css route I can only offer a very vague suggestion.... If you can find an example of using temperature readings to set the background colour of a tile....
, you can use a similar technique, replacing the temp reading reference with your power on and power off states. Then if you find the css to replace the text you can use that in place of the background colour settings in the temperature example. I can probably help more in another 5-6 hrs.
