hello mert, from time to time youtube does some design experiments and enables a different interface for some of its user base - and apparently you're part of it :-) http://googlesystem.blogspot.co.at/2012/09/youtubes-updated-design-experiment.html there's not much you can do about it, since it happen's on youtube's server side. one thing to try if you don't like it is to [[Delete cookies to remove the information websites have stored on your computer|clear cookies for youtube.com]] (however since you apparently have an account there, google might also have tied the experiment to the account and the design might reappear when you log in again).

Thanks for fast answer I deleted youtube.com cookies and yeah,ı get old theme back