Step 1) Login on tumblr and check this site http://www.tumblr.com/themes/recover
Step 2) On the page will appear all the backups that tumblr made of your theme. Max 20 backups. (If you have more than one blog in the account, choose the blog and press View Backups).
If you press Show Changes, will appear some codes, this codes is what you add or remove from your theme. To reverse this changes press Revert.
Note: The red lines are parts what you remove from the code of your theme, and the green lines are what you add.
Step 3) Comment here