There are a couple of background images and styles in Apple's 'Pages' and 'Keynote' softwares, that I was planning to include for my website and mobile app. Would this be intellectual property infringement? I was not able to find this answer in the license agreement of Pages, and Keynote, or may be i missed it since i am not a lawyer. Are these default content opensource?
Almost certainly not.
What pages license are you looking at? Section D starts on page 1: You may not and you agree not to ... create derivative works of the Apple Software or any services provided by the Apple Software.
I would recommend you focus on finding images that are unequivocally granted under creative commons license and perhaps getting a lawyer or at least pay someone to guide you on keeping records of how you source your app and web images.
Just like taxes and accounting - we aren't all accountants, but we need to know when to employ them and the same goes with lawyers.