Before you dig deeper into this, please consider that the change of the layout of a theme in many cases will jeopardize the responsive design.
To adjust the width from 1140px (standard size of the theme) to 980px you can adjust the wrapper ID. e.g.
#wrapper {
width:980px;
}
It will makes the whole blog smaller.
The content areas are calculated in percentage relative to the blog width. Their width also depend on the layout you’ve choosen: One colum in a three column layout is smaller that one column in a two column layout.
In case you’ve choosen 3 columns than:
.blog .twothirdcol {
width:78%;
}
will be the parameter to adjust. You also have to take the sidebar always into consideration. In this example the sidebar must be adjusted to 22%.
.blog .threecol {
width:22%;
}
Hope you get the hook of it.
Cheers,
Alexander @ http://www.mouseclick.com