Can i use 'Pages' default images/background styles for my website or mobile app
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.