moodle/blocks/calendar_month/styles.php

.block_calendar_month .cal_popup_caption,
.block_calendar_month .cal_popup_fg div {
    font-size: 0.85em !important;
}

.block_calendar_month .filters {
    font-size:0.75em;
    padding: 5px;
}

.block_calendar_month .filters table {
    border-collapse:separate;
    border-spacing: 2px;
    padding: 2px;
}

.block_calendar_month .event_global,
.block_calendar_month .event_course,
.block_calendar_month .event_group,
.block_calendar_month .event_user {
    border-width: 2px;
    border-style: solid;
}