CSS

centralize div inside div

        .contentCentralized {
            width: 100%;
            height : 200px; 			//Remove height if you don't want specified height for it
            display: flex;
            flex-direction: column; 	//options : row | row-reverse | column | column-reverse
            flex-wrap: wrap;        	//options : flex-start | flex-end | center | baseline | stretch
            justify-content: center;	//options : start |  center | space-between | space-around | space-evenly
            align-items: center;
        }
        <div class="contentCentralized">
        <h3>Hello Developers.</h3>
        </div>
        

center a dive within another div

#container {
    width: 640px; /* Can be in percentage also. */
    height: auto;
    margin: 0 auto;
    padding: 10px;
    position: relative;
}