Firstly go to the FireFox theme website by clicking the link below.
http://www.mozilla.com/en-US/firefox/personal.html
Once there, there will be a list of theme's which you can activate instead of having the default them all the time.
To do so, choose a Theme you like, once the page is loaded click the green button saying Add to firefox.
Once it is downloaded it will say you have to restart firefox.
When firefox re-open's it will have the new theme should be activated.
If you want to change back to another theme click on Tool's > Add-on's > Theme's and it will again say you have to restart your browser.
For a more easy explanation, Click the link below.
https://www.youtube.com/watch?v=7imnOqIHSz8
http://www.mozilla.com/en-US/firefox/personal.html
Once there, there will be a list of theme's which you can activate instead of having the default them all the time.
To do so, choose a Theme you like, once the page is loaded click the green button saying Add to firefox.
Once it is downloaded it will say you have to restart firefox.
When firefox re-open's it will have the new theme should be activated.
If you want to change back to another theme click on Tool's > Add-on's > Theme's and it will again say you have to restart your browser.
For a more easy explanation, Click the link below.
https://www.youtube.com/watch?v=7imnOqIHSz8