Please bear in mind that Cowbell is still alpha quality software. It may not always be the epitome of stability.
Let's begin with an example of a Cowbell theme file:
/*
* METACITY CSS SUNSHINE THEME
* Author: Thomas Thurman
* Design: Firinel Thurman
* Copyright (c) 2009 Collabora Ltd
* Released under the GNU GPL v2 or any later version at your option.
*/
frame {
-dc-title: "Sunshine";
-dc-description: "A simple orange theme";
-dc-creator: "Thomas Thurman";
-dc-contributor: "Firinel Thurman";
-dc-publisher: "The GNOME Foundation";
-dc-rights: "http://www.gnu.org/licenses/gpl-2.0.html";
-dc-rightsholder: "Collabora Ltd";
-dc-date: "2009-10-28";
-dc-identifier: "http://blogs.gnome.org/metacity/2009/10/29/sunshine/";
border-radius: 3px;
background-color: #c37004;
border: 1px #000 solid;
}
title {
color: #ffce63;
text-align: left;
}
area.titlebar {
border-top: 2px;
border-bottom: 2px;
}
button {
width: 200px;
height: 200px;
background-image: url('blank.png');
background-size: contain;
margin-left-width: 5px;
margin-right-width: 5px;
}
button.menu { background-image: url('menu.png'); }
button.close { background-image: url('close.png'); }
button.maximize { background-image: url('max.png'); }
button.minimize { background-image: url('min.png'); }
button.shade { background-image: url('shade.png'); }
button.stick { background-image: url('stick.png'); }
button.above { background-image: url('above.png'); }

When you run this through Cowbell, the result looks like this.
Download, compile, and install metacity-cowbell.
Copy the contents of the src/themes/Sunshine directory to ~/.themes/Sunshine/cowbell/.
Set the current theme to be "Sunshine" in /apps/metacity/general/theme.
Start Metacity. If you want to ensure the stability of your own system's environment, run Metacity inside Xephyr.