Is there any way to keep the lights from a room shining through walls? There's a chandalier in a closed room, and every area around that room is being lit up by it.

None that I know of so long as you want the doors to be able to open. If you're okay using teleport doors instead of the kind that swing open, you can have each room widely separated within the interior space so that the globe of light within one room simply can't reach far enough to enter another. It isn't necessary to use multiple interior cells, and if you separate the individual rooms only on the Z axis, the mini-map won't be distorted by the increased distances.

It's not an ideal solution. How reasonable it is will depend on how bothered you are by light shining through the walls.

The easiest way would be to lower the Radius of that light.
You can also experiment with Negative Lights. Their IDs all start with dark_ and they give-off black light (shadows)
They can be tricky to get just right, but they add some really neat visuals. I used them in my Sable dragon Inn when I had a similar problem as yours.