You can now customize properties (color, linewidth, point type) of the default sequence of linetypes used in plotting. See `set linetype`. This is normally done in an initialization file, either the system-wide file `gnuplotrc` or a private file `~/.gnuplot`. See `initialization`.
Most terminals now allow you to set an explicit background color for the plot. The special linetype `bgnd` can be used to draw in this color. See `bgnd`.