SquareRoot is a simple metacity that should blend well with any GTK theme.
Feel free to change or improve the theme at your will. Constructive feedback is also welcomed.
To install just
1st. Download and unzip the contents of the .zip file to the /usr/share/themes/ folder
(you will need super user priviledges for this);
2nd. use the Gnome Tweak Tool (Gnome3) to select the Theme - Current Theme (choose SquareRoot);
or use the Appearance Settings of Gnome2 to choose the window borders. Notice that, prior to Gnome 3.6, gnome-shell will need to be restarted for the new window borders to appear. For that, press Alt+F2, write r and hit enter.
Assuming you have wget and unzip on your system, you may also try the following commands to install the theme:
sudo cp -r SquareRoot/ /usr/share/themes/
rm -r SquareRoot
For gnome3, do the following additional steps:
1st. open gnome-tweak-tool
(select SquareRoot in the Themes - Window Theme option)
Press Alt+F2, write r, and hit enter (After Gnome 3.4, this step is not needed).
For gnome2, go to the appearance settings and change the window borders of the current theme. No need to restart anything.