Because Vue scoped CSS works by adding a unique data-v-xxxxx attribute to elements in the current component's template and appending it to every CSS selector. Elements rendered by child components don't get that attribute, so the selectors don't match.

<template>
  <div class="wrapper">
    <DatePicker />
  </div>
</template>

<style scoped>
/* This won't work — .calendar-popup is inside DatePicker's template */
.wrapper .calendar-popup {
  background: white;
}
</style>

Generated CSS looks like .wrapper[data-v-abc123] .calendar-popup[data-v-abc123], but .calendar-popup doesn't have data-v-abc123.

How to fix it

Use the :deep() selector to pierce into child components:

<style scoped>
.wrapper :deep(.calendar-popup) {
  background: white;
}

.wrapper :deep(.date-input) {
  border-color: blue;
}
</style>

Always scope :deep() to a parent class to limit its reach:

<style scoped>
/* Too broad — affects ALL .btn in any child */
:deep(.btn) { background: blue; }

/* Better — only .btn inside .wrapper */
.wrapper :deep(.btn) { background: blue; }
</style>

Exception: the child's root element

A child component's root element IS affected by parent scoped CSS. Vue does this intentionally so parents can control layout (margins, positioning) of their children:

<style scoped>
/* This works without :deep() — targets child's root element */
.date-picker {
  margin-bottom: 1rem;
}
</style>

Old syntax (don't use)

.parent >>> .child { }     /* doesn't work with SCSS */
.parent /deep/ .child { }  /* deprecated */
.parent ::v-deep .child { } /* old Vue 3 syntax */

In Vue 3, :deep() is the only supported syntax.