Themes can be created by setting the style of the line
# viridis
set style line 1 lt 1 lc rgb '#440154' # dark purple
set style line 2 lt 1 lc rgb '#472c7a' # purple
set style line 3 lt 1 lc rgb '#3b518b' # blue
set style line 4 lt 1 lc rgb '#2c718e' # blue
set style line 5 lt 1 lc rgb '#21908d' # blue-green
set style line 6 lt 1 lc rgb '#27ad81' # green
set style line 7 lt 1 lc rgb '#5cc863' # green
set style line 8 lt 1 lc rgb '#aadc32' # lime green
set style line 9 lt 1 lc rgb '#fde725' # yellow