Chrome Extension - Blocking

chrome.webRequest.onBeforeRequest.addListener(function(details) {
    var url = details.url;
    var type = details.type;

    if( url.indexOf('/css/base.css')>=0 ){
        return { cancel: true }
    }

    return { cancel: false }
},
{
    urls: ["<all_urls>"]
    // urls: ["https://*.domain.com/*", "https://testy/*"]
},
["blocking"]
);