How the panes are rendered depends on the skin. The standard DNN skins render them as table cells.

I suppose that you are using one of the standard DNN skins (DNN-Blue or so). Let's say you use the DNN-Blue - Horizontal Menu - Fixed Width skin. If so, goto the Horizontal Menu - Fixed Width.ascx file in your skin directory, in this case Portals\0\Skins\DNN-Blue\ (if it's your first portal. If not, replace the 0 with the appropriate portal ID). In that .ascx file, look for the table cell with the ID="ContentPane". It's look is defined in the CSS class contentpane in the skin.css.

More details about skinning your portal can be found in the Skinning Guide (part of the DocumentatIon pack on the Downloads page).