The best way to fix this is to use addEventListener in settings.js to register the click event handlers on the appropriate elements.
E.g. you can add the following code to your settings.js:

function init() {
  document.getElementById('Button1').addEventListener('click', Save, false);
  document.getElementById('Button2').addEventListener('click', Load, false);
  document.getElementById('Button3').addEventListener('click', Default, false);
}

window.addEventListener('load', init, false);

And remove the onclick=... stuff from the HTML.

Inline scripts are forbidden in extensions by default for security reasons. You should put all your JavaScript code in JS files and not in HTML.
You can read more about the Content Security Policy in extensions (and how to allow exceptions) in the documentation:

• https://dev.opera.com/extensions/tut_architecture_overview.html#permissions_and_privileges
• https://developer.chrome.com/extensions/contentSecurityPolicy