Comment 0 for bug 1356386

Comparing the line throwing the error to the line above it:

gtk-widgets.css
783: box-shadow: 0 1px 0 1px alpha (#000, 0.30);
784: text-shadow: 0 1px 0 1px alpha (#000, 0.30);

Looks like a copy-and-paste error.

According to gnome's specification:
https://developer.gnome.org/gtk3/stable/GtkCssProvider.html#GtkCssSectionType

box-shadow may take up to five arguments:
box-shadow: [ inset ] horizontal_offset vertical_offset blur_radius color;

text-shadow only takes up to four:
text-shadow: horizontal_offset vertical_offset [ blur_radius ] color;