If you send the epub covers with Calibre, their thumbnails will appear identical to kepub thumbnails.
The "white strips" that people (myself included) don't like are more apparent in older firmware, for example see the centre photo in
this old thread. With the current firmware there is a white border around all thumbnails, so the white strips (which only occur when the device generates the cover, never when the cover is sent by Calibre) are not so obvious.