Hi Stephen
Firstly did you mean
#productprint-description table, td, th {
Or did you mean
#productprint-description table td,
#productprint-description table th {and likewise
#productprint-description td, #productprint-description th {
I think the 2nd in each case is what you want.
Make sure you make your edits in the plug-in’s own css file, rather than in the theme’s css file.
However if you want the coloured backgrounds to print, then you have to select this option in the printer dialog box, because most printers will default to not printing solid background colours. This can’t be gotten around from the plug-in because it’s a browser setting.