Chrome Extension - Modify DOM

modify-dom.txt

Background page is kind of like you own mini server - it is a completely isolated page that has nothing to do with pages a user visits. It is used for controlling the rest of extension components as it is always running in background.

Content scripts are pieces of javascript code that are getting injected into actual pages a user visits and can access and modify parent page's DOM.

So to get your extension working you need to remove

<script src="jquery.min.js"></script>    
<script src="content.js"></script>
from background page, and inject jquery as a content script either through manifest:

"content_scripts": [ {
        "all_frames": true,
        "js": [ "jquery.min.js" ],
        "matches": [ "http://*/*", "https://*/*" ] 
} ]
or with chrome.tabs.executeScript:

chrome.browserAction.onClicked.addListener(function(tab) {
    chrome.tabs.executeScript(null, {file:"jquery.min.js"}, function() {
        chrome.tabs.executeScript(null, {file:"content.js"});
    });
});