I was worried about this too as my mice will all be moving outside next week unheated.
Apparently they can survive extreme cold weather sometimes, and even with the temperature falling to -10 degrees they should still be ok if you give them some nesting material, and obviously i should imagine not keeping mice singularly...
Its the heat you need to worry about more, mice don't cope with heat very well.
I disagree completely with what is said here. The house mouse is comfortable at the same temps as we humans and does best at a stable temp of around 72 to 75 degrees Fahrenheit. A house mouse who gets chilled may become sick, or may just seem okay but actually be so compromised as to become sterile. High temps of over 80 F can be fatal, causing convulsions and death. Humidity is important as well; mousies are prone to cold tissue disease, something similar to chillblains in humans, and that can cause reddening and cracking in the tail leaving the mouse susceptable to infection.
In addition, I would add that ex[osure to direct sunlight is unhealthy for mice, as are drafts, and excessive noise and commotion.