Absolute center techniques http://css-tricks.com/centering-css-complete-guide/

absolute-center.css

/*
Cross-browser (including IE8-10)
Más info: http://coding.smashingmagazine.com/2013/08/09/absolute-horizontal-vertical-centering-css/
*/
.absolute-center {
/* height: must be declared */
margin: auto;
position: absolute;
top: 0; left: 0; bottom: 0; right: 0;
}


/*
Variable height content
Won’t work in IE8
Results in blurry rendering of edges and text in some cases
*/
.centering-the-unknown {
  width: 50%;
  margin: auto;
  position: absolute;
  top: 50%; left: 50%;
  -webkit-transform: translate(-50%,-50%);
      -ms-transform: translate(-50%,-50%);
          transform: translate(-50%,-50%);
}

.vertically-centering-the-unknown {
  position: absolute;
  top: 50%;
  -webkit-transform: translate(0,-50%);
      -ms-transform: translate(0,-50%);
          transform: translate(0,-50%);
}

.horizontally-centering-the-unknown {
  position: absolute;
  left: 50%;
  -webkit-transform: translate(-50%,0);
      -ms-transform: translate(-50%,0);
          transform: translate(-50%,0);
}

/* block level element using flexbox */

.parent {
  display: flex;
  flex-direction: column;
  justify-content: center;
}


/* 
Variable height content
Works well cross-browser
*/
/*
<div class="Center-Container is-Table">
  <div class="Table-Cell">
    <div class="Center-Block">
    <!-- CONTENT -->
    </div>
  </div>
</div>
*/

.Center-Container.is-Table { display: table; }
.is-Table .Table-Cell {
  display: table-cell;
  vertical-align: middle;
}
.is-Table .Center-Block {
  width: 50%;
  margin: 0 auto;
}



/*  inline element, single line */

.center-text-trick {
  height: 100px;
  line-height: 100px;
  white-space: nowrap;
}

/* or */

.link {
  padding-top: 30px;
  padding-bottom: 30px;
}


/*  inline element, multiple lines */

.flex-center-vertically {
  display: flex;
  justify-content: center;
  flex-direction: column;
  height: 400px; /* required */
}

/* or ghost element */

.ghost-center {
  position: relative;
}
.ghost-center::before {
  content: " ";
  display: inline-block;
  height: 100%;
  width: 1%;
  vertical-align: middle;
}
.ghost-center p {
  display: inline-block;
  vertical-align: middle;
}

Some more ways for absolute centering. http://digitalfortress.tech/tricks/13-killer-ways-to-center-elements-with-css/