I'm aware of this. You said "core style" which some people refer as the main style.... But I just learned that there is a style called, core (go figure).
However the case, you may still want to rebuild your cache. All styles, even custom styles, use the default style as a point of reference.
The fact that it couldn't find PAGE_CONTAINER suggest there is something not right. Although you could also try to re-apply CORE. And with that said, custom style support should be sought from the developer.