Source

File: wp-includes/class-wp-theme.php

public static function get_allowed_on_network() {
		static $allowed_themes;
		if ( ! isset( $allowed_themes ) ) {
			$allowed_themes = (array) get_site_option( 'allowedthemes' );
		}

		/**
		 * Filters the array of themes allowed on the network.
		 *
		 * @since MU (3.0.0)
		 *
		 * @param string[] $allowed_themes An array of theme stylesheet names.
		 */
		$allowed_themes = apply_filters( 'allowed_themes', $allowed_themes );

		return $allowed_themes;
	}