wrap html content in "page container"

I’m creating a website for a friend using a theme I’m writing, and I’m working in a localhost environment. Right now the front page (home) looks great except for one thing: the major divs that wrap up the page content stop at the top of the page because my page content is not text but is html. I have sliders, scrolling lists and galleries but no long text strings. My html content resides between the header and the footer. I need a white background behind my html content, and the only way I’ve found to get it is to manually extend one of the major divs (like div id=”main”) by giving it a height in pixels. I find this to be impractical because someday someone may want to change the website or content, then suddenly the background becomes the wrong size because it was manually set. Any suggestions on how to wrap my html content in a background-holding div?