Click “Customize” on your Tumblr dashboard. Scroll to the bottom of the Customize pane and click “Advanced Options.” Delete any code in the “Add Custom CSS” section to remove any theme customizations you added. Click the back arrow at the top of the pane to exit the advanced settings. Click “Browse Themes” to search for a new theme or restore your current theme to its original settings. Tumblr displays a list of free and premium themes, along with a thumbnail of each theme layout. Select “All Themes” and then “Free Themes” to see only the basic, free themes. If you already have a theme in mind, click the search icon and enter the theme’s name. To use the original blue Tumblr theme, enter “Redux.” To use the current default theme as of January 2014, enter “Optica.” Other free basic themes include “Default,” “Optimus,” “Papercut” and “Simple Things.” Click a theme to select it and see how it looks in the preview pane. Click “Use” and then “Save” after you find the right theme. If you selected a premium theme, click “Purchase” and enter your credit card or PayPal information. If you reinstall a premium theme you purchased previously, you don’t have to pay again. Click “Exit” to return to your dashboard.
