Changing the colors

HTML5 provides browsers with a full-featured color picker (typically, browsers use the native OS's color chooser). This will be used to allow the user to set the background color of the editor application itself:

The color picker is added to the index.html page with the following HTML:

<input id="pckBackColor" type="color">

The implementation is straightforward as the color picker control provides:

InputElement cp = qs("#pckBackColor");
cp.onChange.listen((e) => document.body.style.backgroundColor = cp.value);

Because the event and property (onChange and value) are common to the input controls, the basic InputElement class can be used.