Did you checked with other theme, is this happening with other theme as well.
If it's only happening on that theme you can be fairly certain it relates to the code in the theme. HTML is loading - since we can see content in your image - so it's a fair assumption that the CSS isn't loading or the file has broken code in place. Have you looked at the files being loaded? Any issues or bugs? Looked at the actual content of the CSS files loading?
Lots of very basic troubleshooting steps to do here but I suppose if you're really stuck consider sharing the url so the forum members can see it.