Sandbox theme: nav-above div floats over border

I’m using the three column basic Sandbox skin for a new blog, mixing in NiftyCorners for a rounded corner effect on the main content div (actual “content” div for those familiar with the theme). Since the content div and page as a whole use the same background color (white), I added a new div inside the content div and then set the inner content div’s background color to white and the content div’s background color to what we want for the border.

However, whenever the “navigation” div is visible, say on the single post page, it displays over the top border, wrecking the rounded corner effect. I think this is due to the nav-next and nav-previous divs being floated, but not sure. Same effect happens at the bottom navigation div too.

Any ideas on how to ‘clear’ this up?

Thanks!