How to make "main" infinitive scrolling in 2011 theme

Hi,
in my 2011 child thème, in the “main” div i added a flash navigation bar at its head and i added also a background which i want “repeat-y”. It would be the background of the “primary” div (content) and of the “secondary” div (sidebar). The problem is that the “main” does not have content, only the navbar, so the background repeats only upto it but does not repeat under the “primary” and “seconday” divs. At the moment i put “main” height to 2000px in style.css but the problem is that when i will have longer content, it does not grow automatically or when there is a short article, its lenght is too long with an empty space.
Can anyone help me to fix it ? Pls do not ask me to change my idea and put the bg in the “primary” and “secondary” divs, my question is : is it possible to repeat a div background with its divs contents…

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Thanks & regards