Ease of altering Css layout

Hello,

Im interested in purchasing this software but I would like to know the following if possible..

I would like to have total control over the layout of my custom theme via the program eg.

If I wish to have a 3 column theme with a header BUT one sidebar goes from the top of the page to the bottom ie also to side of header( so header would be above column1 & 2 )

would this be possible or is it only a set number of grid/css layouts available?