<style>
.Absolute-Center
{
margin: auto;
position: absolute;
top: 0; left: 0; bottom: 0; right: 0;
}
</style>
<div class = "Absolute-Center">
<button id = "button_reg">Регистрация</button>
</div>$( "#nav li a" ).click(function()
{
$( "#nav li a" ).removeClass('active');
$(this).addClass( 'active' );
});
$( "#menu_base" ).click();