To get rid of the border around the content area, you will need to set the border of the left and right column to none.
For example if you select the 3columnfixed layout, then on the Design Tab you would select the leftColumn1 and set the right border to none and then select the rightColumn1 and set the left border to none.
In order to use an external style sheet, on the Output tab, you can specify the style sheet.