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.
- 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