Theme definitely isn’t broken, it just doesn’t include the same CSS styles as your old theme.
This is why it’s usually a good thing to get column codes from plugins, as you can switch themes freely without losing the formatting of your content.
Here’s one plugin I like: https://wordpress.org/plugins/lightweight-grid-columns/
You can also make use of the built in CSS framework in GeneratePress, but doing so will lock you into GP, and if you switch themes again, you’ll need to switch the column codes again. If you’re interested, you can learn more about it here: http://unsemantic.com/
Hope this helps 🙂