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.