Background color change

[John Coppens <john jcoppens com>]

I realize this has probably been asked frequently, but I can't seem to get any
of the solutions to work.

The problem: I compiled Gnumeric 1.12.20 - first time for Gtk3 - and found that
my dark theme produces a fairly dark background, and black text in the cells -
nearly unreadable.

In order to debug this problem, I've been trying to detect which CSS is used for
the cell area, or cells in particular. I ran gtkparasite and found the classes
(GnmSimpleCanvas and friends), but I wasn't able to define CSS (in gtkparasite)
which had any effect on the actual cell contents. I could change some colors,
such as what I suspect is the cell margin.

I have gtk-3.14.8 installed... The theme is FlatStudioDark. I did have to patch
the theme to make GtkGrid and GtkBox transparent (as they are supposed to be,
I believe), else eg. Notebook tabs with an icon+text would render with 
wrong colors.

Of course I can define the foreground color for the text in Gnumeric, but this
does not seem to be an elegant solution for portability.

I'd appreciate suggestions.