Popupcontact form body backgroud

Could you please help me with the code below?
When the popup opens I want the body to blurred or grey transparent with less opacity.

`$(function() {

// contact form animations
$('#contact').click(function() {
  $('#contactForm').fadeToggle();
 
})
$(document).mouseup(function (e) {
  var container = $("#contactForm");

  if (!container.is(e.target) // if the target of the click isn't the container...
      && container.has(e.target).length === 0) // ... nor a descendant of the container

  {
      container.fadeOut();
  }
});

});`

I tried to add $(“body”).css(“opacity”,”0.25″); to change only the opacity, but the form is also changed…