How to extend the width of the customisable slider to fill in the browser window

Hi, I am using the Catch Box Pro Theme and happy with its functionality. However, I do not like the width of the page which does not fill the browser window but has a "background space at the left and right... I wonder how can I change this, or at least extend the header/the slider to reach the two side ends of the browser window? Thank you, Nadia ( brightbraincentre_dot_co_dot_uk ) with very limited programming knowledge!