Reduce Spacing Between Posts & Remove gray striped horizontal bar

On my website, I would like to reduce the white space (vertically) between posts on the grid on my home page / category page / etc and eliminate the gray striped horizontal bar that appears vertically between posts. How can I do this?

I have granted wpmu support access via the plugin, in case that helps... and I'm attaching an image to show what I would like the spacing between posts in the grid to look like.