You are not alone! there are lots of posts around the forum on the wasted space due to the header / footer. Now, these are the keywords you've got to search for: header, footer
At least, there is a way to decrease the footer height via kobopatcher. The header can only be circumvented by not using the kepub-viewer AFAIK (there are two different e-book viewers on the stock kobo: one of them only has a footer, no header and no reading stats per chapter).
https://www.mobileread.com/forums/sho...d.php?t=220171
https://www.mobileread.com/forums/sho...d.php?t=193294