We are glad to know that you considered using Basel for your web-site. I hope you will be happy with it.
It happens because of authorization enabled on your server. When we try to access your SVG icon we get this popup.
Try to disable it in order to make icons work properly.
Unfortunately, SVG icons work not as regular HTML IMG tags and can’t work with this authorization on the server. Anyway, your website will be allowed for public view in the future and then these icons will work.