If you are using X (the windowing system on Unix-like OSes) you might want
to tweak you ~/.Xdefaults to change preference for gnuplot.
Here's my setting:
gnuplot*background: black gnuplot*textColor: white gnuplot*borderColor: white gnuplot*axisColor: white gnuplot*line1Color: red gnuplot*line2Color: green gnuplot*line3Color: blue gnuplot*line4Color: magenta gnuplot*line5Color: cyan gnuplot*line6Color: sienna gnuplot*line7Color: orange gnuplot*line8Color: coral gnuplot*Font: 7x14 #gnuplot*backgroundGray: black gnuplot*textGray: white gnuplot*borderGray: gray50 gnuplot*axisGray: gray50 gnuplot*line1Gray: gray100 gnuplot*line2Gray: gray60 gnuplot*line3Gray: gray80 gnuplot*line4Gray: gray40 gnuplot*line5Gray: gray90 gnuplot*line6Gray: gray50 gnuplot*line7Gray: gray70 gnuplot*line8Gray: gray30 gnuplot*borderWidth: 2 gnuplot*axisWidth: 2 gnuplot*line1Width: 2 gnuplot*line2Width: 2 gnuplot*line3Width: 2 gnuplot*line4Width: 2 gnuplot*line5Width: 2 gnuplot*line6Width: 2 gnuplot*line7Width: 2 gnuplot*line8Width: 2