Change the way negative-numbers class was defined in CSS

After testing with Gtk3.10, needed to change the way the negative
numbers class was defined.

gnucash/gnome-utils/ui/gnucash-fallback.css

@@ -33,13 +33,8 @@
 }
 
 /* Negative value label colors */
-.default-color {
-  color: currentColor;
-}
-
-.negative-numbers {
-  color: rgb(75%, 0%, 0%);
-}
+@define-color default-color currentColor;
+@define-color negative-numbers rgb(75%, 0%, 0%);
 
 /* Change font color by mixing with grey */
 .lighter-grey-mix {

gnucash/gnome-utils/ui/gnucash.css

@@ -2,6 +2,16 @@
    unless they have been named or have style classes added. Only 
    the widget type can be configured unless they are named in code */
 
+
+/* Negative value label colors */
+.default-color {
+  color: @default-color;
+}
+
+.negative-numbers {
+  color: @negative-numbers;
+}
+
 /* Register Cursor padding settings, make sure entry matches sheet.h */
 cursor entry {
   padding: 2px 5px 2px 5px;