WordPress | Add categories programmatically on theme activation

add_the_categories.php

<?php
// Sets the default categories
function add_the_categories() {
    $categories = array(
        'Case Study',
        'Blog',
        'Article'
    );
    foreach( $categories as $category ) {
        $exists = term_exists( $category, 'category' );
        if( !$exists ) {
            // Insert the term if it doesn't exist
            $term = wp_insert_term(
                $category, // the term
                'category', // the taxonomy
                array(
                    'slug' => sanitize_title( $category )
                )
            );
       }
   }
}
add_action( 'after_switch_theme', 'add_the_categories' );