I have got two ways of changing your Facebook profile theme. One is using the Greasemonkey Add-on and the other using the Stylish add-on. Here is how you can change your theme using either of these two:
1)Using Greasemonkey Add-on:
First ensure you are using Firefox browser and then install the Greasemonkey Add-on.
Restart firefox and Visit this page with lots of Themes.(http://userstyles.org/styles/browse/facebook.com)
Click on the favorite theme you like and install the script.
After you load facebook.com you can view the change the Facebook theme.
2) Using Stylish Add-on:
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