Added missing rules for sortable inverted headers

src/definitions/collections/table.less

@@ -930,6 +930,9 @@
 }
 
 /* Inverted */
+.ui.inverted.sortable.table thead th {
+  color: @sortableInvertedColor;
+}
 .ui.inverted.sortable.table thead th.sorted {
   background: @sortableInvertedActiveBackground;
   color: @sortableInvertedActiveColor;

src/themes/default/collections/table.variables

@@ -191,6 +191,7 @@
 @sortableActiveHoverBackground: @transparentBlack;
 @sortableActiveHoverColor: @selectedTextColor;
 
+@sortableInvertedColor: @invertedTextColor;
 @sortableInvertedBorderColor: transparent;
 @sortableInvertedHoverBackground: @transparentWhite @subtleGradient;
 @sortableInvertedHoverColor: @invertedHoveredTextColor;