wp_ajax_install_theme()

function wp_ajax_install_theme() {
	check_ajax_referer( 'updates' );

	if ( empty( $_POST['slug'] ) ) {
		wp_send_json_error( array(
			'slug'         => '',
			'errorCode'    => 'no_theme_specified',
			'errorMessage' => __( 'No theme specified.' ),
		) );
	}

	$slug = sanitize_key( wp_unslash( $_POST['slug'] ) );

	$status = array(
		'install' => 'theme',
		'slug'    => $slug,
	);

	if ( ! current_user_can( 'install_themes' ) ) {
		$status['errorMessage'] = __( 'Sorry, you are not allowed to install themes on this site.' );
		wp_send_json_error( $status );
	}

	include_once( ABSPATH . 'wp-admin/includes/class-wp-upgrader.php' );
	include_once( ABSPATH . 'wp-admin/includes/theme.php' );

	$api = themes_api( 'theme_information', array(
		'slug'   => $slug,
		'fields' => array( 'sections' => false ),
	) );

	if ( is_wp_error( $api ) ) {
		$status['errorMessage'] = $api->get_error_message();
		wp_send_json_error( $status );
	}

	$skin     = new WP_Ajax_Upgrader_Skin();
	$upgrader = new Theme_Upgrader( $skin );
	$result   = $upgrader->install( $api->download_link );

	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
		$status['debug'] = $skin->get_upgrade_messages();
	}

	if ( is_wp_error( $result ) ) {
		$status['errorCode']    = $result->get_error_code();
		$status['errorMessage'] = $result->get_error_message();
		wp_send_json_error( $status );
	} elseif ( is_wp_error( $skin->result ) ) {
		$status['errorCode']    = $skin->result->get_error_code();
		$status['errorMessage'] = $skin->result->get_error_message();
		wp_send_json_error( $status );
	} elseif ( $skin->get_errors()->get_error_code() ) {
		$status['errorMessage'] = $skin->get_error_messages();
		wp_send_json_error( $status );
	} elseif ( is_null( $result ) ) {
		global $wp_filesystem;

		$status['errorCode']    = 'unable_to_connect_to_filesystem';
		$status['errorMessage'] = __( 'Unable to connect to the filesystem. Please confirm your credentials.' );

		// Pass through the error from WP_Filesystem if one was raised.
		if ( $wp_filesystem instanceof WP_Filesystem_Base && is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->get_error_code() ) {
			$status['errorMessage'] = esc_html( $wp_filesystem->errors->get_error_message() );
		}

		wp_send_json_error( $status );
	}

	$status['themeName'] = wp_get_theme( $slug )->get( 'Name' );

	if ( current_user_can( 'switch_themes' ) ) {
		if ( is_multisite() ) {
			$status['activateUrl'] = add_query_arg( array(
				'action'   => 'enable',
				'_wpnonce' => wp_create_nonce( 'enable-theme_' . $slug ),
				'theme'    => $slug,
			), network_admin_url( 'themes.php' ) );
		} else {
			$status['activateUrl'] = add_query_arg( array(
				'action'     => 'activate',
				'_wpnonce'   => wp_create_nonce( 'switch-theme_' . $slug ),
				'stylesheet' => $slug,
			), admin_url( 'themes.php' ) );
		}
	}

	if ( ! is_multisite() && current_user_can( 'edit_theme_options' ) && current_user_can( 'customize' ) ) {
		$status['customizeUrl'] = add_query_arg( array(
			'return' => urlencode( network_admin_url( 'theme-install.php', 'relative' ) ),
		), wp_customize_url( $slug ) );
	}

	/*
	 * See WP_Theme_Install_List_Table::_get_theme_status() if we wanted to check
	 * on post-installation status.
	 */
	wp_send_json_success( $status );
}