Change default background color of PDFs generated via HTML

[marcin-serwin]

When creating PDF output via HTML the background of body element is transparent, so it uses the color of html which is set to #fdfdfd by default. This may potentially lead to wasted printer ink. The introduced fix changes the default color for printing to white while respecting the case of user setting a custom background color.

Steps to reproduce

Run

$ echo foo | pandoc -o foo.pdf -t html

Before and after

The above pictures with increased contrast and lowered brightness

[jgm]

Good improvement!