Note: I am talking about the files in the root of the sample folder. When you compile, they get copied into the subfolder slides-manual/index.html. Everything in that subfolder (in particular css files) is auto-generated. Therefore, you should not edit anything there!
How to figure out which element of the slide corresponds to which section of the css file? Firefox has the “style inspector” tool which reveals names. It is activated by pressing Ctrl-Shift-I. If you press on the “style” tab, it will even show you from which line of the css file came this or that element on the page! (To turn it off press Ctrl-Shift-I again.)
Notice that the width is controlled by the width parameter of .maincolumn.