We will be at AKL the first week of December. For those who have traveled to WDW in December, is it reasonable to plan on swimming in the evening at all, or is it typically too cold/dark?
It is always possible. The pools are heated so the issue is only getting out and running. I would suggest taking a change of clothes with you and a quick run to the showers (closer than the entrance to the building/walk to a room).
As said above, the pools are heated. I've been in December when it's been 85 and I've been there when it's been 50; FL winter weather is almost impossible to predict more than a day or two in advance.
It all depends on your tolerance for cold - the pools themselves are heated and great in any temperature essentially. It's getting out that's a problem when it's chilly. We've always had fun swimming even when it's 50-something, but we're more cold tolerant than many & at 50 getting out of the pool is painful. In early December, I'd expect it to be relatively warm though - not sure we've seen many times it's dipped below 70 on any of our December trips...
The pools are well lit so no issues with night time swimming beyond temperature tolerance. In December I'd expect you'll be fine, especially if you have some flexibility to skip a particularly cold day if the weather doesn't cooperate.