Tumblr is like a marriage between microblogging, social networking and traditional blogging, giving you the option to post text, photos, video, links, chat conversations, audio and quotes. Tumblr's social networking aspect comes from the Dashboard, where you can view posts from other user on Tumblr that you follow. Tumblr does not give you the option to set a custom dashboard background, however you can change the standard blue background to the Limited Edition Black Dashboard.
Navigate to Tumblr's website and log in to your account.
Click the "Account" link in the top-right corner of your Dashboard, and then select "Preferences" from the drop-down menu.
Scroll to the bottom of the page to the "Dashboard" heading. Check the "Enable Limited Edition Black Dashboard" box.
Click the "Save preferences" button to apply your changes and return to your Dashboard. You can go back to the traditional blue Dashboard at any time by unchecking the box.