If you're busy customizing your theme HTML and something goes wrong, try visiting http://www.tumblr.com/themes/recover - here you can find recent backups of your theme code, so you can easily revert back to your last working version!

When you make changes to the theme HTML, Tumblr no longer automatically updates your theme when we add new features and improvements. If you'd like to update to the latest version of the theme or start afresh, you can reset the theme. To do this, open the Browse Themes menu on your Edit Theme page, find the theme, and hit Use – this will grab the latest version and reset any changes you made.