Bug 796669 - Dark Theme Text Colors Hard to Read

Only add the register-foreground class when using Gnucash built in
colours. When this setting not used, the foreground colour by default
will be what ever the theme has set and will be down to the user to
over ride along with the other register colours.

gnucash/register/register-gnome/gnucash-sheet.c

@@ -2451,8 +2451,11 @@ gnucash_get_style_classes (GnucashSheet *sheet, GtkStyleContext *stylectxt,
         field_type -= COLOR_NEGATIVE;
     }
     else
-        gtk_style_context_add_class (stylectxt, "register-foreground");
-
+    {
+        if (sheet->use_gnc_color_theme) // only add this class if builtin colors used
+            gtk_style_context_add_class (stylectxt, "register-foreground");
+    }
+    
     switch (field_type)
     {
     default: