Change to conform to civil twilight time.

See Wikipedia[1] for more details.

[1] http://en.wikipedia.org/wiki/Twilight#Civil_twilight
Switch mid-way through Civil Twilight time

1. Switching the background at sunset is too early. There is still
   plenty of light in the sky.

2. Switching the background at the end of civil twilight time is
   too late. It's long since become dark outside and my display is
   still bright and white.

3. I've noticed that I tend to close the blinds at work about half-way
   through civil twilight time. Let's 'close the blinds' on our
   background at the same time. Just right.