Simple fix: return COLOR_WIDGET_FOREGROUND if the GtkCssProvider is 0
(i.e., if a foreground color has not been set).

Tested on GTK3.20, 3.18, 3.16, and 3.14. No additional AllNonBrowser
JUnit test failures.

Change-Id: I497f81fc4ef7b3c962f6573abe2cc101a85d7981
Signed-off-by: Eric Williams <ericwill@redhat.com>