Update themes to accommodate C# grammar change (#192854)

See https://github.com/dotnet/csharp-tmLanguage/issues/290

extensions/theme-defaults/themes/dark_plus.json

@@ -77,6 +77,7 @@
 				"source.cpp keyword.operator.new",
 				"keyword.operator.delete",
 				"keyword.other.using",
+				"keyword.other.directive.using",
 				"keyword.other.operator",
 				"entity.name.operator"
 			],