In data/edit-page.js
You can't have a property called background-color
the dash messes it up. You have to put it in quotes. So:
$("body div").css("visibility", "hidden");
$("body").append( $("#addon_hide_page") );
var styles = {
width: "100%",
height: "100%",
"backgroud-color": "gray" ////////////////fix here
}
$("body #addon_hide_page").css(styles);
The best way to debug is to use browser console.
Set the developement preferences (install this addon: DevPrefs) then press Ctrl+Shift+J
and watch the error messages. Use console.log console.info console.warn and console.error to log messages into Browser Concole. Do console.log(objectName) and in browser console you can click on that object and it will show you whats inside it.