Not so smart, those +SmartThings, huh:

"Unfortunately there is not a way to change the temperature readings from Fahrenheit to Celsius, but we do hope to support that functionality very soon."

I guess in this case the user needs to be smart and do the conversion in his head, interpolating based on 15 => 59, 20 => 68, 25 => 77, and my all-time favourite: 28 => 82.  The lounge is currently 20 degrees.