The best and easiest solution is to use a custom css class to over-ride the default like below and use the following to clear the padding and margins:

.cal_Theme1 * {padding:0px;margin:0px;}

 The styles below can go in a linked stylesheet of in your document head.

/* Calendar css */
.cal_Theme1 * {padding:0px;margin:0px;}
.cal_Theme1 .ajax__calendar_container {background-color: #e2e2e2;width:192px;border:solid 1px #cccccc;}
.cal_Theme1 .ajax__calendar_header{background-color: #ffffff; margin-bottom: 4px;}
.cal_Theme1 .ajax__calendar_title,
.cal_Theme1 .ajax__calendar_next,
.cal_Theme1 .ajax__calendar_prev{color: #004080; padding-top: 3px;}
.cal_Theme1 .ajax__calendar_body{background-color: #e9e9e9; border: solid 1px #cccccc;}
.cal_Theme1 .ajax__calendar_dayname{text-align:center; font-weight:bold; margin-bottom: 4px; margin-top: 2px;}
.cal_Theme1 .ajax__calendar_day{text-align:center;}
.cal_Theme1 .ajax__calendar_hover .ajax__calendar_day,
.cal_Theme1 .ajax__calendar_hover .ajax__calendar_month,
.cal_Theme1 .ajax__calendar_hover .ajax__calendar_year,
.cal_Theme1 .ajax__calendar_active{color: #004080; font-weight:bold; background-color: #ffffff;}
.cal_Theme1 .ajax__calendar_today{font-weight:bold;}
.cal_Theme1 .ajax__calendar_other,
.cal_Theme1 .ajax__calendar_hover .ajax__calendar_today,
.cal_Theme1 .ajax__calendar_hover .ajax__calendar_title{color: #bbbbbb;}

ASP Code:

this is how you reference the style you created above in the asp page

<asp:TextBox ID=”F1_I2″ runat=”server” CssClass=”bigInput”>
</asp:TextBox>
<cc1:CalendarExtender ID=”F1_I2_CalendarExtender” CssClass=”cal_Theme1″ runat=”server” TargetControlID=”F1_I2″ Format=”dd/MM/yyyy”>
</cc1:CalendarExtender>