Your container element (in your example, the container) must be position: relative.
The item to center (your text block) must be position: absolute. Now, you can move this item around in relation to the parent. Start with one of the position presets (top center, likely) and set the top to 50%. That will center it vertically. Now, set the left and right margin to auto. That will center it horizontally.
You can see an example of this technique in the demo kit:
https://preview.webflow.com/preview/demokit?preview=72388b288a26874be319ca1f7412d4eb
Another technique would be to use flexbox, but that requires custom CSS at this time.