Yes there is a way to reduce the margins. It works with KF8. But I cannot say if it works with KFX.
Code:
html {
margin-right: -45px;
margin-left: -45px;
}
Put that at the top of the CSS and it will reduce the margins. -45px is for a 6" 300dpi screen. Feel free to adjust as needed.
You do have to remove the DRM from KF8 eBooks that have DRM.