Arrow to go to the top of the page blacks out

Hi,

I am setting up my blog and I am using Foodiz free as a theme. I have noticed that the arrow on the right on the page, the one that is needed to go to the top of the page, has blacked out. I had already installed the theme on a different site and the same had occurred. I have not been able to identify the issue that causes it.

The page I need help with: [log in to see the link]