I thought it would be through my new custom template so I copied the css file from the classic folder and added it to my template folder - it did change the box heading background colours and a few other things but
not the width and other positionings of the site - where is this controlled from?
In general, this request is too broad but I can assist with the only targeted item you referenced - -
A link to your development site is always the best option with definte explanation of what you are attempting
Find in your stylesheet.css the following and you can adjust the highlighted value in px, %, or em
Code:
#mainWrapper {
background-color: #ffffff;
text-align: left;
width: 750px;
vertical-align: top;
border: 1px solid #9a9a9a;
}