wp-includes/blocks/calendar/style.css

.wp-block-calendar{
  text-align:center;
}
.wp-block-calendar td,.wp-block-calendar th{
  border:1px solid;
  padding:.25em;
}
.wp-block-calendar th{
  font-weight:400;
}
.wp-block-calendar caption{
  background-color:inherit;
}
.wp-block-calendar table{
  border-collapse:collapse;
  width:100%;
}
.wp-block-calendar table:where(:not(.has-text-color)){
  color:#40464d;
}
.wp-block-calendar table:where(:not(.has-text-color)) td,.wp-block-calendar table:where(:not(.has-text-color)) th{
  border-color:#ddd;
}
.wp-block-calendar table.has-background th{
  background-color:inherit;
}
.wp-block-calendar table.has-text-color th{
  color:inherit;
}

:where(.wp-block-calendar table:not(.has-background) th){
  background:#ddd;
}