How to remove prev/next article nav border image

Hi,

I’m trying to remove the border image line with the word NAVIGATION in the center of it, that is just above the previous/next post navigation links. The only thing I can manage is to remove the whole navigation thing, with:
.post-navigation {display: none; }

instead of just the border. I believe the image is called border.png and I have found all bits of css with that in it and tried a few and put border: none; or border-image: none; in it’s place, but it’s not worked. Clearly I’m on the wrong track, so any help would be appreciated.

My site is https://www.helenajohnson.net

Thanks