Just used Tumblr’s redesigned customization interface; it seems like an upgrade and looks like it may be based upon ACE (Ajax.org Cloud9 Editor.) The remnants of Mozilla Bespin got merged into ACE, and the editor seems okay, although I did run into a bug where the sidebar was taller than my display, so when I scrolled to the bottom of the Edit HTML pane with arrow keys, I had to do some Web Inspector hackery to get back to the buttons up to (running Chrome 14.)