I do understand you. Do you have any suggestions?
No, no good suggestions at least, yet . What I think might work is simply having alternating background colors for the <li id="post-X"> elements (e.g. two different shades of lightgray or alternating between lightgray and lightblue, something in that area; the current background colour of the thread page isn't white, so there is some room to play with), however, just doing that would look not very good without some further alterations.
Visibly connecting the post container with the user info container might solve this 'feeling' as well, but would also not look good.
I have to play around with it a little bit myself, to come up with something sensible.