It took me a long time to figure out that you could really customize every aspect of your Tumblr blog. +Tumblr
As it turns out, there are a few different methods that you can use to customize the sidebar. The easiest method is to use the “Description” field via your Tumblr Customize page. If you’re logged into Tumblr you can customize your blog by clicking the “Customize” button while viewing your blog.
For detailed instructions on how to customize your Tumblr using the Description field as well as by entering custom-HTML code, please check out this article on HubPages: