// contentscript.js:
chrome.extension.sendRequest({}, function(responseText) {
    // .... ???? Function logic ????
    // Let's assume that the responseText contains the CSS url.
    var currentTheme = responseText + '.css';
    var style = document.createElement('link');
    style.type = "text/css";
    style.rel = "stylesheet";
    style.href = currentTheme;
    (document.head||document.documentElement.firstElementChild).appendChild(style);
});


// background page:
chrome.extension.onRequest.addListener(function(request, sender, callback) {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        if(xhttp.readystate == 4) {
             callback(xhttp.responseText);
             xhttp.onreadystatechange = xhttp.open = xhttp.send = null;
             xhttp = null;
        }
    }
    xhttp.open("post", "http://localhost:8888/g_dta.php", true);
    xhttp.send(request.data);
});