<script>
    function OnClientLoad(sender, args) {
        var selectedItem = sender.get_selectedItem();
        if (selectedItem) {
            $telerik.$(selectedItem.get_element()).addClass("rcbSelected");
        }
    }

    function OnClientSelectedIndexChanged(sender, args) {
        $telerik.$(sender.get_dropDownElement()).find(".rcbSelected").removeClass("rcbSelected");
        var selectedItem = sender.get_selectedItem();
        if (selectedItem) {
            $telerik.$(selectedItem.get_element()).addClass("rcbSelected");
        }
    }
</script>

<style>
    div.RadComboBoxDropDown .rcbSelected {
        color: red;
        background-color: lightgreen;
    }
</style>
<telerik:RadComboBox ID="RadComboBox1" runat="server" RenderMode="Lightweight" OnClientSelectedIndexChanged="OnClientSelectedIndexChanged" OnClientLoad="OnClientLoad" MaxHeight="150">
    <Items>
        <telerik:RadComboBoxItem Text="Item 1" />
        <telerik:RadComboBoxItem Text="Item 2" />
        <telerik:RadComboBoxItem Text="Item 3" Selected="true" />
        <telerik:RadComboBoxItem Text="Item 4" />
    </Items>
</telerik:RadComboBox>