fix(VCalendar): use theme background color for categories

[KareemDa]

Description

fixes #14433

Motivation and Context

How Has This Been Tested?

background color was set to white without getting sass varaibles og theme:calender

Markup:

<template>
  <v-app id="inspire">
      <v-calendar type="category"  />
  </v-app>
</template>

<script>
export default {
  mounted () {
    this.$vuetify.theme.dark = true;
  },
}
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)