It's using the standard asp.net calendar control so the easiest thing to do is create a .skin file in your App_Themes/< site > directory called default.skin and add a <asp:calendar>
node to it. Then add the <OtherMonthDayStyle>
property and set the attributes like so <OtherMonthDayStyle BackColor="Transparent" ForeColor="#CCCCCC" />
. Then in your calendar webpart define the skin name, in my case it's "EventCalendar".
Here's an example of the skin file:
<cms:CMSCalendar runat="server" SkinID="EventCalendar" Width="100%">
<TodayDayStyle BackColor="#F8F7F4" BorderWidth="1" BorderColor="#585880" ForeColor="#585880" />
<OtherMonthDayStyle BackColor="Transparent" ForeColor="#CCCCCC" />
<SelectedDayStyle ForeColor="#6464FE" BackColor="transparent" />
<NextPrevStyle Font-Bold="True" ForeColor="#585880" BorderColor="Transparent" BackColor="Transparent" />
<DayStyle ForeColor="#000000" BorderColor="transparent" BackColor="Transparent" Height="50px" />
<SelectorStyle Font-Bold="True" ForeColor="#696969" BackColor="#F8F7F4" />
<WeekendDayStyle Font-Bold="False" ForeColor="#000000" BackColor="transparent" />
<DayHeaderStyle Font-Bold="True" ForeColor="#585880" BackColor="Transparent" />
</cms:CMSCalendar>