Hello,
You can add a custom class in the Appearance tab, you can set it to "StyleCalendar," for example.
then you can style whatever you want, like the below, in your style file.
.StyleCalendar {
background-color: rgb(104, 14, 165) !important;
color: #e414a5 !important;
}
.widget-calendar .rbc-off-range-bg {
background-color: rgb(104, 14, 165) !important;
}