jQuery toggle CSS?

For jQuery versions lower than 1.9 (see https://api.jquery.com/toggle-event):

$('#user_button').toggle(function () {
    $("#user_button").css({borderBottomLeftRadius: "0px"});
}, function () {
    $("#user_button").css({borderBottomLeftRadius: "5px"});
});

Using classes in this case would be better than setting the css directly though, look at the addClass and removeClass methods alecwh mentioned.

$('#user_button').toggle(function () {
    $("#user_button").addClass("active");
}, function () {
    $("#user_button").removeClass("active");
});