Tumblr allows you to install a new theme and change the appearance of your blog with just a few mouse clicks. After you have saved a theme to your blog, however, there is no simple way to revert back to the original theme included with Tumblr. Tumblr's default theme is called "Redux," and by installing this theme from the theme list, you can restore your blog to its original appearance.
Open your Tumblr dashboard by visiting Tumblr.com in a Web browser and logging in with your e-mail address and password.
Click the name of your blog on the dashboard, and then click "Change Appearance."
Click the "Theme" tab at the top of the page.
Scroll down to the "Free Themes" section, and click the "Redux" theme.
Click "Save + Close" at the top of the page.