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

Deletes the old theme during an upgrade.

Gets the WP_Theme object for a theme.

Turns on maintenance mode before attempting to upgrade the active theme.

Turns off maintenance mode after upgrading the active theme.

Initializes the upgrade strings.

Initializes the installation strings.

Checks if a child theme is being installed and its parent also needs to be installed.

Don’t display the activate and preview actions to the user.

Install a theme package.

Upgrades a theme.

Upgrades several themes at once.

Checks that the package source contains a valid theme.