Hmm. I made a quick new theme with no edits. From there, I left Opacity at 90 (to show the borders in the discussion list); however, upon entering the thread, the message block's color isn't shown, despite putting the background as "pink". Message Block User seems to work fine though. Below are two attachment. First is one with Opacity 90. Second is Opacity Zero, but the borders between threads in discussion list disappears.