anchor.diff:  
     @override
     Widget build(BuildContext context) {
  +    final ThemeData themeData = Theme.of(context);
       return new Flexible(
         child: new Material(
  -        elevation: 24,
  -        color: Colors.grey[700],
  +        color: themeData.backgroundColor,
           child: new Column(
             children: <Widget>[
               new CalcKey('\u232B', calcState.handleDelTap),
  
positive.code:         children: <Widget>[
            // Give the key-pad 3/5 of the vertical space and the display 2/5.
            new CalcDisplay(2, _expression.toString()),
            new KeyPad(3, calcState: this)
          ]
        )
  
negative.code:       // and the op keypad have sizes proportional to their number of
        // columns.
        flex: 3,
        child: new Column(
          children: <Widget>[
            new KeyRow(<Widget>[
              new NumberKey(7, calcState),
              new NumberKey(8, calcState),
              new NumberKey(9, calcState)
            ]),
            new KeyRow(<Widget>[
              new NumberKey(4, calcState),
              new NumberKey(5, calcState),
              new NumberKey(6, calcState)
            ]),
            new KeyRow(<Widget>[
              new NumberKey(1, calcState),
              new NumberKey(2, calcState),
              new NumberKey(3, calcState)
            ]),
            new KeyRow(<Widget>[
              new CalcKey('.', calcState.handlePointTap),
              new NumberKey(0, calcState),
              new CalcKey('=', calcState.handleEqualsTap),
            ])
          ]
        )
      );
    }
  