I imagine bunnies are quite pleased when winter comes around and it's gets around 18-20oC indoors - much more comfortable for them! A house bunny shouldn't need anything extra unless you're living in a very cold house. Anything in double digits should be fine for the average healthy rabbit, and about 18oC minimum for ones with any health issues that could be exacerbated. If you're not sure, pop on your winter coat, gloves, hat and scarf - if you still feel a bit chilly sitting around doing nothing then turn up the heating a bit.