Changed as suggest.

Terminal.Gui/Views/Button.cs

@@ -156,10 +156,10 @@ namespace Terminal.Gui {
 			}
 		}
 
-		Rune _leftBracket = new Rune ('[');
-		Rune _rightBracket = new Rune (']');
-		Rune _leftDefault = new Rune ('<');
-		Rune _rightDefault = new Rune ('>');
+		Rune _leftBracket = new Rune (Driver.LeftBracket);
+		Rune _rightBracket = new Rune (Driver.RightBracket);
+		Rune _leftDefault = new Rune (Driver.LeftDefaultIndicator);
+		Rune _rightDefault = new Rune (Driver.RightDefaultIndicator);
 
 		internal void Update ()
 		{