I say 'mostly' mild because it can get cold in Paraguay. It even FEELS colder than it actually is as most homes do not have heat and are not carpeted. It feels like you are in a refrigerator when the temperature drops. Everybody just puts on lots of layers of clothes.
If you are fortunate enough to have a fireplace then you can always sit by the fire on a cold evening.
The following is an experpt from my friend's blog you can check out at http://jungle-hut.blogspot.com/2009/07/things-i-have-recently-learned.html "Remember that when it is really cold, one should not use electric heaters as they are dangerous to your health. Heaters are to only be used when it is cool, but not too cold. If it is really cold, just wear more clothes." I love Paraguay!