Comment 0 for bug 1597526

GTK 3.20 was released in March and made substantial changes to theming. It would be great if Lubuntu's themes were ready for it. Ideally, Lubuntu will be ready for GTK 3.20 for Ubuntu 16.10. See bug 1576576.

See the Ubuntu's Desktop's PPA for yakkety:
https://launchpad.net/~ubuntu-desktop/+archive/ubuntu/gtk320

https://lists.ubuntu.com/archives/ubuntu-desktop/2016-June/004805.html

Ubuntu GNOME offers a GNOME3 Staging PPA if you want to test the rest of GNOME 3.20. Since the yakkety series already has GTK 3.21, you may want to just use the xenial series of this PPA.

https://launchpad.net/~gnome3-team/+archive/ubuntu/gnome3-staging

Some resources for the theme conversion:
https://github.com/shimmerproject/Greybird/issues/120#issuecomment-224121206