I make a row with three columns in it. I want the middle one to have a backgroundcolor that goes from the row above to the row under it. But when one of the columns left or right gets longer (higher) then the middle one, the color of the middle column stops where the column stops. To put it another way: how do I make the height of a column the full span between two rows?
Is it possible to fix this without editing the css in an external editor?

If I understand correctly, you are having a problem where uneven column heights are allowing the row behind the columns to show. It looks something like this.
http://uscni.com/coffeecup/forums/column-height-uneven.png

To correct the issue, you just need to set a minimum height on the columns so that they are all the same height. (You don't have to give all 3 columns the same class name, but I do it so I can set the height once and it sets all three.)
http://uscni.com/coffeecup/forums/column-height.png

The place where you will need to pay attention is when you start moving the slider and the elements within the column get longer forcing the height to exceed possibly the minimum height producing the same effect. For that you can set a maximum height and then use Overflow (in the Position section) to deal with the text when it exceeds the height of the column.

That simple, thanks!