Can´t make the header image fullscreen

Hi, I have a question about the appearance of the header image on the Adventurous theme. I want the image to go completely out to the edges of the screen even if I zoom out the web browser, but I can´t find any settings for that. Right now, it appears the gray areas in the side of the header when the site is displayed on big computer screens, and I think it´s not look so good. Here is a screenshot so you can see what I mean: http://i.imgur.com/k8JCs8I.png It is an important and decisive setting if I will buy this theme's pro version or not. Otherwise, I think this theme looks very good visually, very clean and modern. Hoping for a helpful answer Sincerely robin