title: centering a div vertically inside another div with 100% height
body: <p>I want to center a div inside a div that has 100% height. basically the main div comes before the site. i upload a image so you can understand better what i want to do.</p>
  
  <p><img src="https://i.stack.imgur.com/RogTf.jpg" alt="enter image description here"></p>