Hi there,
This is a small styling conflict with your theme, it applies a padding to all the <select> elements in your site, overwriting our default styling.
You can easily fix this by going to WP Admin -> Appearance -> Customize -> Additional CSS and add in this code:
.wpsbc-container .wpsbc-calendars .wpsbc-calendar .wpsbc-calendar-header .wpsbc-select-container select {
padding: 0 10px !important;
}