Zen Suite contains simple themes and visually attractive, this suite support GTK+ 2, GTK+3, and Xfce. Zen suite has eight variants with different color schemes (Zen, Zen-bicolor, Zen-blua, Zen-griza, Zen-nigra, Zen-nokto, Zen-verda, Zen-papero). This suite has no metacity by itself, so I added another great looking metacity to all variants and xfwm theme to its latest version for trusty 14.04. It is specially designed for Xfce environment, Xfce widgets and notifications stuff included.
You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change themes.