Boomerang is a set of two cool GTK3/GTK2 themes by ghogaru. These themes support both Unity and Gnome Shell and come with dark panel/menu and light panel/menu variants.
The theme pack requires Unico GTK3, Murrine GTK2 and Pixbuf GTK2 engines. While Unico and Murrine are installed by default in Ubuntu, you can install Pixbuf GTK2 engine by running the command below:
sudo apt-get install gtk2-engines-pixbuf
Now just grab the theme pack from here and extract the archive into usr/share/themes. You can use Gnome Tweak Tool to switch themes.
1. ALT + Right-click on the panel and select properties.
2. Click on the 'Background' tab and select the option 'Background image'.
3. Set the 'panel_bg.png'/'panel_bg_dark.png' (included in the archive) as the background image.
If you are using ArchLinux or Fedora, you can find installation instructions in INSTALL file included in archive.