(Source: monzo12782, via suspend)
1. On the Customize screen turn on the Fix Theme option. 
2. You can also try to turn off HTTPS/Encryption in your Tumblr Blog Settings. https://www.tumblr.com/settings/blog/your-blog
3. If you customized the HTML code of the theme you may have broken it so I suggest redownloading or using Tumblr's Theme Recovery.