How to apply CSS for button ?

Hi

I have a button like this 

<script type="text/javascript">
          function confirmationDel()
          {
            var txtRef = document.getElementById('<%= txtTranRef.ClientID %>').value;
            var returnData=true;
            if(txtRef == "")
            {
                  alert("Please capture transaction reference number.");
                  txtRef.focus();
                  returnData = false;
             }
             if(txtRef.length < 10 && txtRef.length > 0)
             {
                  alert("Transaction reference number length must be 10.");
                  txtRef.focus();
                 returnData=false;
             }
             else
             {
                var confirmMsg = confirm("Are you sure you want to delete this tran?");
                if(confirmMsg)
                {
                    returnData= true;
                }
                else
                   returnData=false;
             }
           return returnData;
        }</script><asp:Button ID="btnDelete" runat="server" Text="Delete" class="alt_btn" OnClick="btnDelete_Click"
                     OnClientClick="return confirmationDel();" UseSubmitBehavior="false" />

class="alt_btn" is not being applied on this button. If i remove UseSubmitBehaviour="false" then it is applying. How to solve this issue?

Thanks