You can enable here black background of the drawing. The normal background color is white.
You can use this function to enhance visibility of some drawings. However please note that the black background affects logical graphics operations, for example color merging. It means, that some files can be displayed incorrectly on the black background. It is recommended to use black background only for simple vector drawings without raster images.
In this mode all vector entities drawn by the black color are changed to white color, all other colors are unchanged.
The key shortcut for switching the black background on/off is Ctrl-K. You can also add a corresponding button to the toolbar.
See also Inverse Colors