If you really want to let the cell style displaying behind the selected row, please return the 'class' name instead of the inline style in your 'styler' function. The code below shows how to achieve this functionality.
<style>
.datagrid-row-over .c1{
background:#eaf2ff;
}
.datagrid-row-selected .c1{
background:#ffe48d;
}
.c1{
background:#ffee00;
color:red;
}
</style>
<script type="text/javascript">
function cellStyler(value,row,index){
if (value < 30){
return {class:'c1'};
}
}
</script>