As AlbertaCowboy said, it is coded in the CSS. If you are sideloading the books, then his suggestion of using calibre to convert and change the CSS is a good one.
If you are reading a book purchased from Kobo and downloaded to the device, it is a little different. You can change some CSS using an override file on the device. See
https://www.mobileread.com/forums/sho...d.php?t=218814 for details.