Add PHP 8.4 and 8.5 compatibility

[SamueleMartini]

Summary

Adds PHP 8.4 and 8.5 compatibility to the module. Changes are fully backward compatible with PHP 8.2 and 8.3.

Changes

• Plugin/ThemeAdminhtmlSwitcherPlugin.php: added explicit nullable types (?string $area = null) to comply with PHP 8.4 deprecation of implicit nullable parameters; added : array return type; promoted property visibility to protected; extracted config path to a class constant.
• Model/Config/Source/AdminThemeList.php: promoted $themeList property visibility to protected; extracted magic string Magento/backend to a class constant; simplified title computation with a ternary; trimmed redundant PHPDoc descriptions.
• composer.json: added explicit PHP constraint ~8.2.0||~8.3.0||~8.4.0||~8.5.0.
• CHANGELOG.md: new 1.2.1 entry dated 2026-04-21.

PHP 8.4 rationale

PHP 8.4 deprecates implicit nullable parameter types — any parameter with a non-null type hint and a null default must be declared explicitly nullable (?Type). The upstream Magento\Theme\Model\View\Design::setDesignTheme($theme, $area = null) is not typed, but $area in the plugin's beforeSetDesignTheme is a string-or-null, so an explicit ?string is the safe and future-proof signature. $themeId is kept as mixed because the underlying method accepts both string theme paths and ThemeInterface objects.

PHP 8.5

No code changes were required specifically for 8.5 — the same typed signatures are forward compatible.

Suggested release

I'd suggest tagging this as 1.2.1 (patch — compatibility fix, no functional changes).

Test plan

• composer install on a Magento 2 / Mage-OS instance running PHP 8.4
• composer install on a Magento 2 / Mage-OS instance running PHP 8.5 (when available)
• Verify admin theme selection in Stores > Configuration > Advanced > Admin > Admin Design lists all installed admin themes
• Verify the selected admin theme is applied in the backend

cc @rhoerr for review.

[rhoerr]

Thank you