GNOME 3’s default theme is a pale, elegant affair. Granted the window controls are a bit on the, shall we say beefier side, but it’s still a great theme that is very easy on the eye.
If you like the theme and want to use it in Ubuntu, but don’t want the hassle that comes with it adding the GNOME 3 PPA, you now can thanks to a GTK2 port of the theme dubbed ‘Aldabra’.
It’s a fairly faithful affair of Adwaita, although though lessened in use on non-GNOME 3 systems due to the lack of inclusion of the GNOME 3 icon theme.
The fact it’s such a stellar effort was intentional; Aldabra was designed to adequately mimic the look of Adwaita so as to “…provide visual consistency between GTK2 and GTK3 applications.”