Stilo is a pack of clean, minimalistic, yet stylish GTK themes for the GNOME desktop. It consists on 2 main themes, Stilo and Stiloetto, each with light and dark variations. A GNOME Shell theme is also available.
Both Stilo and Stiloetto use gray with bits of blue, the difference being that Stilo is completely flat and square, while Stiloetto uses a slight gradient for header bars, and slightly rounded corners for application windows, and elements like buttons and drop-downs.
These are Stilo and Stiloetto GTK themes (each available in light and dark versions):