I’m not sure how much of a difference this makes but the produce page for the theme says:
HTML5 | CSS3 and Proper Code
The latest standards that make it possible, Google can now detect outdated websites, so it’s better to be on the latest. The theme is properly coded to avoid plugin conflicts and is safe. Iconic One Pro doesn’t use any vulnerable scripts like timthumb.
Kindly upload a logo and re-test, your site will then be validated as proper HTML 5 for sure. The demo site is still running the old version.
The other minor issues that can be addressed, will be addressed in the upcoming version. Please note that the validator tools are meant for only guidance. For eg. check the sites like apple.com, microsoft.com even they won’t pass because the validator tools are outdated.