gtk3-engines-oxygen software package provides Oxygen widget theme for GTK3-based applications, you can install in your Ubuntu 14.04 LTS (Trusty Tahr) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install gtk3-engines-oxygen
gtk3-engines-oxygen is installed in your system.
Make ensure the gtk3-engines-oxygen package were installed using the commands given below,
$ sudo dpkg-query -l | grep gtk3-engines-oxygen *
You will get with gtk3-engines-oxygen package name, version, architecture and description in a table
| Distro | Ubuntu 14 04 lts trusty tahr |
| Section | GNOME |
| Package | gtk3-engines-oxygen |
| Version | 1.3.5-0ubuntu1 |
| Description | Oxygen widget theme for GTK3-based applications |
| Sub Section | GNOME |