New themes and moving from GNOME/GTK 3.10 to 3.18 means two good years of Mint 18.x ahead.
The newly released Mint 18 is a major upgrade. Not only has the Linux Mint project improved Mint’s dueling desktops (Cinnamon and MATE), but the group’s latest work impacts all underlying systems. With Mint 18, Linux Mint has finally moved its base software system from Ubuntu 14.04 to the new Ubuntu 16.04.
Upgrading to the latest long-term support (LTS) release of Ubuntu means, as with the Mint 17.x series, the Mint 18.x release cycle is now locked to its base for two years. Rather than tracking alongside Ubuntu, Mint 18 and all subsequent releases will stick with Ubuntu 16.04. Mint won’t necessarily get as out of date as Ubuntu LTS releases tend to by the end of their two-year cycle, but this setup does mean nothing major is going to change for quite a while.
Read more at Ars Technica