You can easily backup Tumblr theme by copying the code from the 'Edit HTML' view and saving in a Notpad file or as a '.txt' file. Making a backup copy of your Tumblr theme is useful for allowing others to use your theme too. If you like to experiment with your theme, and you botch it up, you can always paste the backup code over your botched up code to restore the original theme code. Follow the instructions below to backup or in other words, make a copy of your theme code.

Backup Tumblr Theme

1. Log into your Tumblr theme.
2. Click on the 'Preferences' icon.
   
3. Click the 'Customize your blog' button.
   
4. Click on the 'Edit HTML' button on the left side of the screen.
   
5. Highlight all the markup code in the HTML window and copy it (Ctrl-c) .
   
6. Open a Notepad and paste (Ctrl-v) the entire HTML code into it and save it on your computer. You're done, that's how you backup Tumblr theme.

Note: It's very common for people to copy their Tumblr theme code to a '.txt' file and give it to friends to use as their theme. This is perfectly fine if your theme is free. If you paid for your theme, then chances are you've only purchased a 'one-time-use license'. Giving the theme to others to use may violate this agreement so make sure you read the purchase agreement.