How to change the border of main page

Could someone please help me in changing the look of the border on my website. The website is https://www.leysinpark.ch.
As you can see, there is a thin black border around the page. I would like to make a shadow around it and make the corners a little round. Maybe make the line a little thicker.

Also, where do you change the background color? Can you add a cool background of somesort.