I have black leather and in the summer (more like from spring till fall) I use a cover that consists of small wooden pieces, so it breathes. In the winter it gets cold but gets pleasant really fast with the heater. Of course it is a question of preferences; I turn off the heater after a minute or two but my wife keeps it on low for hours.
However cold it is, the leather does not get noticeably stiffer.
Hope this helps...
- Peter