Use the same tile you are already using on the floor in the herringbone pattern, but run it in straight vertically like boards. Make sure you have plenty if not extra lighting as your color scheme is a bit dark.

Though grey [shower rocks] it seems rustic, more than the other elements -- does that fit your house in general? .... you don't want something more similar to the tiles? what about penny type? I could be way off...

Haha- funny you ask. My husband LOVES the river rocks. This is a basement and the whole look will blend throughout ... I had considered getting rid of the river rocks but he just loves them so I gave up. Thank you!!!

Has your husband actually stood on those rocks in his bare feet as for shower walls the same as the floor would be best this is not a huge space probably and too many different finishes just looks bad . IMO there are really no rustic elements in the space and FYI barn doors are not sound proof so be aware and I would do a simple pattern on the floor and shower