If your question type is column oriented then you can choose alternate color under format tab. Else, you can try the code I use. Put this code under footer.
<style type="text/css">
.grid_c1.alt_color1
{
background-color: #E5ECF9;
}
.grid_c1.alt_color2
{
background-color: #E5ECF9;
} </style>