<select id="settingBackground">
    <option name="default">Default</option>
    <option name="thankshaking">Background 2</option>
</select>

$(document).ready(function () {
    $('#settingBackground').on('change', function () {
        if (this.value == "Default") {
            // Change background
            $('body').css("background", "#1e8cd4");
            $('body').css("color", "#fff");
        } else if (this.value == "Background 2") {
            // Change background
            $('body').css("background", "url('http://i.imgur.com/cV7PKqh.jpg') no-repeat center center fixed");
            $('body').css("background-size", "cover");
            $('body').css("color", "#000");
        }
    });
});