Mediterranean themes offer light and dark options for your Ubuntu 14.04 system. Here's how you can install them.
1. Open a terminal window.
2. Type in the following commands then hit Enter after each.
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install mediterranean-theme
According to Noobs Lab:
This theme suite contains 12 variants dark/light for GTK and Xfce, in this series all the themes are same but with different color variants and some bitmaps.More at Noobs Lab
Additional Ubuntu resources