On the LiSTNR app
To view all categories, go to the podcast tab at the top of the home screen, scroll down and you will find a section that contains some of the categories as well as an option to view them all.
Alternatively you can go to the Discover tab in the navigation, and you will see the list of categories.
On the web player - play.listnr.com
To view all categories, go to the Podcast page in the navigation and scroll down to the Categories row.