place-content is a shortcut for both justify-content and align-content, and place-items is a shortcut for both justify-items and align-items. So, either place-content: center or place-items: center should do the job. They operate the same for one item. They will behave differently if you have multiple items in your container, and Kevin Powell explains the difference in a video called
Place-content vs. place-items | What's the difference to vertically & horizontally center?.
If you want to put the CSS on what you want centered instead of on its container, you can use place-self instead. All of these should work whether the container is a grid or a flexbox. I appear to have used both place-items and place-self in the CSS rendering of the square shape in Game Courier.