While Tumblr lets you customize your blog by installing free and paid themes, it provides no option for removing an installed theme. You can, however, revert back to your default Tumblr theme, Optica, by manually searching for and applying the theme via Tumblr's Themes panel or Theme Garden.
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Click the Account icon on your Tumblr dashboard, and then click Settings.
Select a blog from under the Blogs section.
Click the Edit Theme button next to Website Theme.
Click Edit Theme on the left sidebar.
Click the magnifying glass icon, enter Optica into the search bar, and then click the Optica search result. To apply the theme, click Use.
Click Save to finalize the changes.
Tumblr Theme Garden
Visit the Tumblr Theme Garden. Search for the Optica theme, and then click the Optica search result.
Click the Install button to start applying the theme.
Select the blog to reset, and then click Install to apply the theme.