[css] What CSS selector can be used to select the first div within another div

If we can assume that the H1 is always going to be there, then

div h1+div {...}

but don't be afraid to specify the id of the content div:

#content h1+div {...}

That's about as good as you can get cross-browser right now without resorting to a JavaScript library like jQuery. Using h1+div ensures that only the first div after the H1 gets the style. There are alternatives, but they rely on CSS3 selectors, and thus won't work on most IE installs.