Upgrade WordPress to PHP 8.3

Project: Upgrade WordPress site to PHP 8.3 (custom discontinued theme compatibility)

My site www.NORGESHUS.eu
currently runs on PHP 7.4 and now shows occasional deprecation warnings. I need a safe upgrade to PHP 8.3 while keeping our custom theme behavior and design exactly as-is (templates, shortcodes, widgets, admin options, etc.).

We use the Hometown WordPress theme, which is discontinued and not supported, so this requires manual PHP 8.3 compatibility refactoring.

Audit the custom theme (and any theme-related custom code) for PHP 8.x / 8.3 compatibility (deprecated functions, strict type issues, parameter order changes, etc.).

Refactor/patch anything that breaks under PHP 8.3.

Update WordPress core / plugins / bundled libraries only if required for PHP 8.3 compatibility, and document all version changes.

Provide a backup + rollback plan before deployment.
Zero/near-zero downtime deployment plan. We don’t have a staging server: you may clone the site to a temporary environment for testing, but it must be private/access-restricted and deleted after completion.
Final validation on live: no fatal errors and no new warnings/deprecations in logs, and confirmed front-end/back-end parity with the pre-upgrade site.

Deliverables Live www.NORGESHUS.eu
running on PHP 8.3, with the custom theme fully functional.
A short upgrade report including:
what was changed and why
versions updated
test checklist performed
future maintenance notes
(optional) a diff / list of files modified