Applying different style to center column in multi post pages only

Atahualpa has been excellent so far, and I've found answers on these forums to most of my questions, but here's one I couldn't find an answer to: is it possible to apply a different style to posts on the multi post pages (eg. when a category is selected) only? The formatting as it is for the home page and single post pages is fine, but for multi post pages I would like the excerpts to be a lot tighter together vertically (ie. reduced top/bottom padding/margin for body, header, footer, etc). With the current formatting, there is a lot of empty space on multi post pages between the headline and the body, and the body and the next post.

I imagine that I will have to add some PHP into The Loop, and a CSS insert.

Here is an example of a multi post category page with too much vertical spacing in it:
http://www.maijagraham.com/?cat=13

Thank you.