Plotted SVG has no background even if some parts of graph are painted with explicit color.
This differs from PNG output where the background is plotted.
(Note that both formats support transparency.)
Picking up any foreground color without knowing background can lead into unreadable image.
OTOH, there can be valid use cases when we suppose light/dark enough background with carefully selected painting colors making acceptable contrast. (This allows e.g. custom background add in postprocessing.)
The superior SVG supports so called `currentColor'. Thus proper usage of currentColor foreground and undefined/unfilled background can produce totally color independent pictures.
So I suggest:
* gnuplot should provide option to drive not-plotting explicit background.
* gnuplot could provide option to produce color independent pictures by rigorous usage of currentColor and avoidance of other colors.