Set the correct permissions for WordPress files

WordPress permissions should be set to 755 for directories and 644 for files.
The entire set must have the same user/group as the server.

chown www-data:www-data -R *
find . -type d | xargs chmod -v 755
find . -type f | xargs chmod -v 644

Reproduce this customization today

The principle of the hook remains useful, but the administration screens and WordPress APIs have evolved since the publication. Test the code in a staging environment with the exact version of the site's core, theme, and plugins.

• Save the files and the database before making any changes.
• Control user capabilities, nonces, input validation, and output escaping.
• Place customizations in a dedicated plugin or a child theme to preserve updates.

Reference: official WordPress plugin development manual.