Cartes du Ciel is free software released under the terms of the
GNU General Public License
When you set your chart to use a Alt-Az coordinate system together with the usage of a automatic background colour, the sky background will display a lighter colour, related to the sort of twilight.
Astronomical dusk is the period of time during which the center of the sun is lower than 18º degrees of arc below the horizon for your observation position. For those periods, SkyChart will usually display a dark background, except when the Moon is above the horizon.