Datepicker UI in DNN 6 uses Telerik RadCalendar control. The theme of the control is using "default". This website outline the details of how to customize the look and feel of the datepicker to fit in your site design.

You can see more Telerik "skin" options and how to customize them here.

<div class="dnnForm" id="form-demo"> <fieldset> <div class="dnnFormItem"> <asp:Label runat="server" CssClass="dnnFormLabel" AssociatedControlID="StartDatePicker" Text="Pick a date" /> <dnn:DnnDatePicker runat="server" CssClass="dnnFormInput" ID="StartDatePicker" /> </div> </fieldset> </div>