$('body').css("background-color","orange");

<style>
background: #4096ee; /* Old browsers */
background: -moz-linear-gradient(top,  #4096ee 0%, #7abcff 100%); /* FF3.6+ */
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#4096ee), color-stop(100%,#7abcff)); /* Chrome,Safari4+ */
background: -webkit-linear-gradient(top,  #4096ee 0%,#7abcff 100%); /* Chrome10+,Safari5.1+ */
background: -o-linear-gradient(top,  #4096ee 0%,#7abcff 100%); /* Opera 11.10+ */
background: -ms-linear-gradient(top,  #4096ee 0%,#7abcff 100%); /* IE10+ */
background: linear-gradient(to bottom,  #4096ee 0%,#7abcff 100%); /* W3C */
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#4096ee', endColorstr='#7abcff',GradientType=0 ); /* IE6-9 */
</style>

$('body').css({"background-color":"orange","background": "-moz-linear-gradient(top,  #4096ee 0%, #7abcff 100%")});

$('body').addClass("style_on");//для применяем класс с новыми стилями
$('body').removeClass("style_on");//убераем класс с новыми стилями