In reply to Liesbeth.

What you did is exactly what you have to do. That's how the css box model works. When you set the width it only sets the width of the content itself. However what you see as the whole box (in this case the column) also includes paddings and borders.

Here's a post I wrote for InstantShift on the css box model. There's a section in that post about calculating widths and heights of css boxes.

The main thing to know with these columns is if you increase padding you'll need to decrease the width, just like you did.