The code below shows how to custom the checkbox styles.
<style type="text/css">
.datagrid-cell-check input{
width: 15px;
height: 1px;
position: relative;
}
.datagrid-cell-check input::before{
content: '';
position: absolute;
width: 15px;
height: 15px;
left: 0;
top: 0;
background: #999;
}
.datagrid-cell-check input:checked::after{
position: absolute;
box-sizing: border-box;
width: 15px;
height: 15px;
top: 0;
padding-left: 4px;
color: #fff;
background: #555;
content: 'x';
font-size: 11px;
}
</style>