WordPress will be able to view the calendar in the widget.
graftee has already put a color on the calendar in the style.css.

Screenshot of the General of the Settings

there is a ‘Week Starts On’ in WordPress General Settings page.
It has a with a color that corresponds to both the setting of the ‘Sunday’ or ‘Monday’.
In other words, even attaches color to the calendar not have to do anything.

Screenshot of calendar widget of graftee theme