<div class="square">
<div class="square1">45</div>
</div>
.square {
height: 150px;
width: 150px;
background: #ccc;
display: flex;
flex-direction: row;
flex-wrap: nowrap;
justify-content: center;
align-items: center;
align-content: stretch;
}
.square1 {
background: orange;
width: 90px;
height: 90px;
-o-transform: rotate(45deg);
-ms-transform: rotate(45deg);
-moz-transform: rotate(45deg);
-webkit-transform: rotate(45deg);
transform: rotate(45deg);
display: flex;
flex-direction: row;
flex-wrap: nowrap;
justify-content: center;
align-items: center;
align-content: stretch;
}