Bringing up this old thread to ask for this decision to be reconsidered. I find the sidebar in the screenshots you posted to be completely comprehensible. It's clear from the expand buttons that States and Trackers are major categories and I feel there's really no need to redesign the entire side bar in order to make the client fully themeable.
If you still don't want this to be themeable in the traditional sense (i.e. in gtkrc), could the fg/bg colors of the category lines possibly be made into gtkui.conf options? The default could be the current colors and aesthetes could change it to match their gtk theme if they so desire. The current colors are very ugly in dark themes like mine :/
Deluge
