Follow this small tutorial to fix that!
Firefox Fix:
1.) Install the Stylish Plugin. You can download it here: https://addons.mozilla.org/en-US/firefox/addon/stylish/2.) Go to http://userstyles.org/styles/79542/youtube-centered-dec-6-2012
3.) Click Install with Stylish.
4.) And you're done! Go check out your new centered Youtube!
Chrome Fix:
1.) Google 'Stylish Chrome' or go to https://chrome.google.com/webstore/search-extensions/stylish and add it to Chrome.
2.) go to http://userstyles.org/styles/79542/youtube-centered-dec-6-2012
3.) click the Install with Stylish button.
4.) And you're done! Go check out your new centered Youtube!