Unfortunately it's currently impossible to make an empty seat disappear at stars. It uses the same image for empty seat, as it does for take seat. If you make the empty one disappear, you can never take a seat at a table. The best you can do is soften it, or make it 60% opaque or so.
I made a No Images version of Hyper-Simple, which does a pretty good job of making the empty seats hard to see. I used to this theme a lot, but eventually wanted to be able to see the empty seats better, so I would know if a table was breaking down.