Hello, Abigail!
Actually, that is not the embedded video. It’s just the linked thumbnail video.
If you’d prefer the video itself (rather than a linked thumbnail video) to be embedded in your WordPress page or post then try to follow the instructions below:
https://embedpress.com/docs/add-wistia-videos-wordpress/
As this is not a problem in the theme and we do not personally use Wistia, unfortunately, I cannot help you with this.
You may want to ask wistia support staff for help.
Please contact us again after embedding your video if you need help centering it.
Regards