background.js

// Listens when new request
chrome.webRequest.onHeadersReceived.addListener(function(details) {
  for (i = 0; i < details.responseHeaders.length; i++) {

    if (isCSPHeader(details.responseHeaders[i].name.toUpperCase())) {
      var csp = details.responseHeaders[i].value;

      // append "https://mysite.com" to the authorized sites
      csp = csp.replace('script-src', 'script-src https://mysite.com');
      csp = csp.replace('style-src', 'style-src https://mysite.com');

      details.responseHeaders[i].value = csp;
    }
  }

  return { // Return the new HTTP header
    responseHeaders: details.responseHeaders
  };
}, {
  urls: ["https://github.com/*"],
  types: ["main_frame"]
}, ["blocking", "responseHeaders"]);


function isCSPHeader(headerName) {
  return (headerName == 'CONTENT-SECURITY-POLICY') || (headerName == 'X-WEBKIT-CSP');
}