There are separate device stylesheets for ePub and KePub books, but they are dynamic, created in memory from fragments with values that depend on which options you have selected on the device.
Currently the only way to change the device stylesheets is by
patching the firmware. In earlier firmware versions there was a user stylesheet named kepub-book.css that was used for KePub books, but that feature was removed some time back (somewhere around firmware 3.2.0 I think.)
If you have a book where you can't change the font face, then I think that must be an ePub book. From what I know there is no way the publisher can prevent the font being changed in a KePub book. If you sideload the book in KePub format instead then I think you will get something a bit closer to what you see in the Kobo app.