title_body: How to remove white margins from ePub in Google Play Books
upvoted_answer: You can use Sigil to change the margins and border by changing the style file. 
  If you have many files to change, you might be better of using Calibre's EPUB to EPUB conversion and setting the margins for the output file to your liking.
  
  If you don't have access to these tools and know about CSS, you can also unzip the EPUB files, change the style sheet by hand and rezip the files back into an EPUB. (This sounds more complicated than it is.)