Some accounts have a redesigned Material Design interface. How can I enable it for a specific account?
The new design is not tied to accounts, it's tied to cookies. You need to set either a specific parameter in the
PREF cookie or set the
VISITOR_INFO1_LIVE cookie to a specific value, but which value it is exactly changes all the time as YouTube is trying out new iterations.
If YouTube thinks your browser supports the new design, you can enable it on https://youtube.com/new.
At present, using the following URL will take you to the new design youtube.com/new.