Tumblr is Moving Things. Again.
You can’t access the “Customize” page like normal through the sidebar of each blog. You now have to go to “Settings” at the top of the page - the icon that looks like a cog - and then on the left hand side you have all your blogs listed. Click on it, then you get the link to the customize page.
It’s just easier if you bookmark this URL from now on:
http://www.tumblr.com/customize/
http://www.tumblr.com/customize/side-blog-name