Source

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

public function delete_old_theme( $removed, $local_destination, $remote_destination, $theme ) {
		global $wp_filesystem;

		if ( is_wp_error( $removed ) ) {
			return $removed; // Pass errors through.
		}

		if ( ! isset( $theme['theme'] ) ) {
			return $removed;
		}

		$theme      = $theme['theme'];
		$themes_dir = trailingslashit( $wp_filesystem->wp_themes_dir( $theme ) );
		if ( $wp_filesystem->exists( $themes_dir . $theme ) ) {
			if ( ! $wp_filesystem->delete( $themes_dir . $theme, true ) ) {
				return false;
			}
		}

		return true;
	}