The most important issue in cold weather is to frequently check the water - and replace with a new bowl or bottle if frozen. Feeding some food that contains water like freshly picked grass, leafy herbs or a small amount of veg is useful too. You can also add some droplets of water to your leaves.
It was very topical as today as it was the coldest day for working in a shed! We tend to feel it when its heading sub zero at night and our minds immediately jump to the rabbits and guinea pigs. They have their small (so easily warmed) areas and to help this we have banked up with straw as well as hay. Straw is a natural bedding and warmer than hay. You can stuff a top box or den dig or look out tower full and let them organise it afterwards.
To increase warmth we have made an extra double insulating layer to capture warmth before it escapes on the roof of the top box. We always did for the guinea pigs but we have treated the rabbits to the same - Teddy appears young but is ten years old after all!
We're keeping our water outside rather than in the top box as although this would be warmer the guinea pigs will spill it all over the bed and the rabbits won't even find a bowl in there! We want to encourage them to move which also keeps them warm so any treats including pellets are offered anywhere in their run so they have to seek it out and get warm looking for it. We usually combine with some fresh hay and clump it on a seat or the floor but never use a bowl.
Fresh picked herbs and grass can be added to the hay wall in the top box or hay tubes in the runs.
The clear front of the top box (when uncovered) is excellent for allowing shy winter sunshine to send a ray of warmth inside. This is why the clear dig is quite popular as it is a small area which quickly heats up with just a little sun. The dig is great with soil in summer but more effective with straw and hay in winter.
Our bunnies are not alone or we may add a warmer. As it is we rely on them to cuddle up together at night. Free exercise in the day is also good as we know ourselves - once we start walking or running we want to take the layers off. A rabbit can't take his winter fur off so bringing him suddenly inside would be traumatic. It would be like wearing a fur coat in your living room!
If you are worried about cold bunnies we suggest you make a good draught free small area for your bunny to go to preferably with two exits. They still like to hear the outside world so ideally connect to the outside not inside especially if they are already used to being out. It would be odd to come inside because the sounds of nature would be missing. Everything a rabbit can react to makes him move and use his senses. This is good in cold weather and good all the time for it is what he is naturally interested in.