£15/hour, soz but it’s my time, it’s my work.
I’ll give you a little advice from what I looked at,
Your max width is set to 1260px, change it to 100%, (leave header width 100%, also and your menu)
Add another div wrapper for content , make it any width you like and add margin: 0 auto;
You will need a little default css, to get a clue where to modify.
I use chrome, right click on element -> inspect element, read css for element that you want to change, also look at associated elements e.g. parent element that you want to change, which might conflict.
I suggest to leave content width 100% as well.
Your header image => add div with background image => background-size:100% 100%; background-postion:center center;