We checked your SVG icons in the header and see that they have the wrong markup. Try to re-upload the correct icons and test how they work. You can download SVG icons here for example https://www.flaticon.com/
We don’t know but it looks like SVG icons were broken before you uploaded them to your website. Try to open original files in the browser separately and see if they work.
Do you mean that all SVG icons were broken right after dummy content import? In this case, we would need to try to import the dummy content from scratch. But in this case you will lose all the content and settings on your website.