All of Tumblr is now served over HTTPS by default. As a result, if you’ve recently tried to update your theme’s custom HTML or blog description, you might have received an error message asking you to try again using only HTTPS URLs.
If you’re the type of person who knows what that means, then you can upload new HTTPS assets. Head to your Theme Customization page, click “Edit HTML,” and then click “Theme assets.”
Otherwise, you can fix this issue by installing a new theme for your blog.
Read more about HTTPS and Tumblr here.