Get unwanted border arround the webpage every time I reload it

I am new to Wordpress. Now I am working with website that is written on Wordpress. Every time I open the site there is white border around the whole website. It appears every time page reloads for some seconds and shrink text content of the page. Then it disappears and page content fits to is usual size. I have no idea what cause it to appear. I would be glad to any suggestions.