Tumblr’s theme recovery page is one of the most tragically overlooked features on the platform. Though it’s certainly skewed toward “power users” (PC slang, lol), Recovery is a powerful tool in undoing mistakes of many sizes with minimal stress.
Theme Recovery is exactly what it sounds like. Tumblr stores up to 20 versions/revisions of your blog’s theme, and allows you to retrieve them if you accidentally fly your HTML ship into an asteroid. It’s important to note that this only applies to direct HTML changes—that is, Tumblr saves versions when the HTML itself has been changed, not every time you change a setting like header text or other “secondary” theme options.
For those of you who like tweaking code but occasionally goof it up, Recovery is absolutely indispensable.