An old theme on a new core: compatibility is more than whether the layout breaks

Situation

A legacy WordPress theme can look perfectly fine on a newer core while producing warnings in the background or failing only on less frequently visited page types. For Columbia Games, visual continuity mattered, but it mattered just as much not to ship hidden PHP-compatibility problems into production.

Approach

Beyond the common pages, I checked category, search, archive and custom-template routes together with the theme's admin interface. I watched the logs separately because several older API usages first appear only as warnings before becoming real failures on a later PHP version. I treated rendered output and runtime health together as the acceptance boundary.

Outcome

The upgrade remained compatible beyond screenshot level. We also surfaced areas that still worked but had already become technical debt. Those could be moved into a separate backlog without unnecessarily blocking the core upgrade itself.