Add this code to the Custom CSS field on the Style tab of your Event Calendar widget’s settings:
.global-styles,
.eapp-events-calendar-dates-calendar .CalendarDay__selected_start,
.eapp-events-calendar-dates-calendar .CalendarDay__selected_end {
border-color: gray;
background-color: gray !important;
color: yellow;
}
.global-styles,
.eapp-events-calendar-dates-calendar .CalendarDay__selected_span {
border-color: yellow;
background-color: yellow !important;
color: gray;
}