Forcing YT to play videos in the new player

How can I get Youtube to play all videos in the new transparent player?

Apparently it used to be possible to do this by going to the bottom of a YT page and clicking "try something new" and that would take you to where you could turn it on, but you can't do it there any more.

I saw that there are some extensions for some browsers that allow you to edit a single cookie which makes YT play in the new player always, but I'm on Opera so I'm not sure whether such a thing exists for Opera, and, furthermore, I'd first like to see if there's some way to do this without installing further extensions.

YouTube (and many other sites) are running segmented test runs of various things. For example the layout of the site etc.
The transparent player you are referring to has been standard for me for more than a few months, with no extensions touching anything YT related.
I'm not sure if it helps reinstalling the browser, but I've noticed a lot of things change when I reinstall my computer - even when using same gmail/YT account.
Make a backup for your bookmarks etc and try reinstalling your browser, that's what I'd try.

Well, that's a little drastic , there must be a simpler way to do this. There used to be.