You can achieve this using regular JS DOM or jQuery code from within the Extension Page Scope. So, to get you started, in the extension.js file you could use something like:
appAPI.ready(function($) {
$('<div id="my-toolbar"></div>').
css({
"background-color":"wheat",
height:"50px",
left:0,
position:"fixed",
top:0,
width:"100%"})
.appendTo('body');
});
[Disclosure: I am a Crossrider employee]