Ok so based on your advise, I pasted the above code in Home/Index.cshtml
It messes up the whole header as it makes everything at the top 100% width.
How can I make header and menu content in the center how it was before and only make slider 100% width?
That's why I said you have to add CSS to pull the content back in the middle that you want from the top. For example, the header-menu class goes all the way across. In order to pull the menu back to the middle you have to add CSS styles to the div element directly after header-menu which is "top-menu" since it is calling the menu.
To pull the menu to the middle you would have to add the following css to the top-menu class "max-width:960px;margin-right:auto;margin-left:auto;" Also, you can change the .header-menu class to take off the rounded corners.
The same thing applies to the header...look at the element that contains the content you want and then add CSS constraints to set the max width and right and left margins to auto. Hope this helps!