The theme uses modules to display posts and each module can display the post category tag. In the following image you can see how the tags look on a module on the front-end:
Multiple categories case:
If the post is included in more than one category the first one will be displayed. If you want to show a specific category, you have to set a primary category from the post editing section.
How to Enable the Category Tag?
Using the Flex blocks/ Flex grids, you can set the category tag directly in the block settings.