It's because of the custom CSS with syntax error in your Header. I commented it out and the issue's fixed.
In order to avoid display issues, before inserting CSS anywhere in your site, please post them in a CSS validator tool like http://csslint.net/. It's not just in our products. It's applicable to website development in general.
I also previously listed:
I could have done better by including examples such as Global CSS, Content CSS, Header CSS, Footer CSS and Element CSS. There are still lots of areas where you can insert CSS but those are common areas in our products.
Hope that helps.