1. On the Customize screen turn off the Use default mobile theme option under Advanced Options. 
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.