<div class='customAlert'>
  <p class='message'></p>
	<input type='button' class='confirmButton' value='Ok'>
</div>
  
<input type='button' class='rab' value='Raise alert'>
@import url(https://fonts.googleapis.com/css?family=Noto+Sans);

body{
  background: darken(#04B486, 30%);
  font-family: 'Noto Sans', 'Helvetica';
}

.customAlert{
	
  display: none;
  position: fixed;
  max-width: 25%;
  min-width: 250px !important;
  min-height: 20%;
  height: 200px;
  left: 50%;
  top: 50%;
  padding: 10px;
  box-sizing: border-box;
  margin-left: -12.5%;
  margin-top: -5.2%;
  background: #088A68;
  
  @media all and (max-width: 1300px){
    .message{
      font-size: 14px !important;
    }
    input[type='button']{
      height: 15% !important;
    }
  }
  
  .message{
    padding: 5px;
    color: white;
    font-size: 14px;
    line-height: 20px;
    text-align: justify;
  }
    
  input[type='button']{
    position: absolute;
    top: 100%;
    left: 50%;
    width: 50%;
    height: 36px;
    margin-top: -45px;
    margin-left: -25%;
    outline: 0;
    border: 0;
    background: #04B486;
    color: white;
    &:hover{
      transition: 0.3s;
      cursor: pointer;
    	background: lighten(#04B486, 5%);  
    }
  } 
}
      
.rab{
  width: 200px;
  height: 30px;
  outline: 0;
  border: 0;
  color: white;
  background: darken(#04B486, 5%);
}
      
@keyframes fadeOut{
	from{
  	opacity: 1;
  } 
  to{
    opacity: 0;
  }
}
    
@keyframes fadeIn{
	from{
  	opacity: 0;
  } 
  to{
    opacity: 1;
  }
}
View Compiled
var currentCallback;

// override default browser alert
window.alert = function(msg, callback){
  $('.message').text(msg);
  $('.customAlert').css('animation', 'fadeIn 0.3s linear');
  $('.customAlert').css('display', 'inline');
  setTimeout(function(){
    $('.customAlert').css('animation', 'none');
  }, 300);
  currentCallback = callback;
}

$(function(){
  
  // add listener for when our confirmation button is clicked
	$('.confirmButton').click(function(){
    $('.customAlert').css('animation', 'fadeOut 0.3s linear');
    setTimeout(function(){
     $('.customAlert').css('animation', 'none');
		$('.customAlert').css('display', 'none');
    }, 300);
    currentCallback();
  })
  
  $('.rab').click(function(){
    alert("If you think about anything, you are actually doing a recursive function which resolves your neurons into a deep pi calculation. You are then executing about 4294 threads in your brain, which do a parallel computation.", function(){
      console.log("Callback executed");
    })
  });
  
  // our custom alert box
  setTimeout(function(){
    alert('You are probably reading this alert box and have no clue why the heck you are even reading it, well guess what, the moon in reality is nothing else than a big ass pokemon, floating in space.', function(){
        console.log("Callback executed");
      });
  }, 500);
});

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js