Greetings,
Just ran into another nifty Courier issue. In main.css, folder icons appear to be assigned to Sent and Trash via the truncated mailbox name (i.e. sent, trash). When running Courier, the truncated names are "inboxsent" and "inboxtrash", respectively. I had to modify the CSS to reference those names, instead of the defaults, in order to make the icons appear.
Dave