Greybird is the default theme of Xubuntu. I personally really like it, it is very polished. The new version of the theme (Greybird 3.20.0) now supports GTK 3.20 as well and it has been ported to SASS.
Greybird features
- Full GTK3 support with consistent theming for GTK2
- High accessibility and readability
- Support for Firefox, Chromium and LibreOffice
- Application specific theming for several Xfce components, including Thunar, Xfce Terminal, panel and application switcher
- Special theming support for indicators in Ubuntu
- Compiz/Gnome support with the emerald, metacity and mutter themes
