Try this :
Add SpecialDays as shown below in order to highlight current day in calendar popup of RadDateTimePiker.
<telerik:raddatetimepicker id="RadDateTimePicker1" runat="server" culture="English (United States)" xmlns:telerik="#unknown">
<calendar>
<specialdays>
<telerik:radcalendarday repeatable="Today" itemstyle-backcolor="Bisque" />
</specialdays>
</calendar>
</telerik:raddatetimepicker>