A useful tool to customize gtk theme is gtk-theme-config.

You can see here the greybird gtk theme wich has a blue scheme by default turned to purple and the gtk theme config at the bottom right corner.

Greybird gtk them wich has a blue scheme by default.

For install it in Saucy 13.10 and Trusty 14.04:

sudo apt install gtk-theme-config

For previous ubuntu releases:

sudo add-apt-repository ppa:shimmerproject/ppa
sudo apt update
sudo apt install gtk-theme-config

Source:

Important

Notice this will not work with the new default themes in ubuntu as explained here:

If all you need is more green, just install one of the themes available at ubuntu-art.org:

The simplest way is maybe to install the color-varied versions of Radiance and Ambiance: Ambiance & Radiance Colors.

sudo add-apt-repository ppa:ravefinity-project/ppa
sudo apt-get update
sudo apt-get install ambiance-colors radiance-colors

And then change the theme with Unity Tweak Tool.

sudo apt install unity-tweak-tool

As for the icons, we can change only the color if we want, while keeping the same theme (in fact creating a color-variation version of the default) with:

sudo apt remove folder-color folder-color-caja folder-color-common

sudo add-apt-repository ppa:costales/folder-color

sudo apt update

sudo apt install folder-color folder-color-caja