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/usr/share/themes/oxygen-gtk/gtk-3.0/gtk-csd.css is in gtk3-engines-oxygen 1.3.5-0ubuntu1.

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The actual contents of the file can be viewed below.

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/*******
 * CSD *
 *******/
.titlebar {
    border-radius: 7px 7px 0px 0px;
}

.tiled .titlebar {
    border-radius: 0;
}

.maximized .titlebar {
    border-radius: 0;
}

/* this is the default titlebar that is added by GTK
 * when client-side decorations are in use and the application
 * did not set a custom titlebar.
 */
.titlebar.default-decoration {
    border: none;
    box-shadow: none;
}

.titlebar .titlebutton {
    background: none;
    padding: 5px 5px 6px 5px;
    border-radius: 3px;
}

.window-frame {
    border-radius: 4px;
    border-width: 1px;
    border-style: solid;

    box-shadow: 0 2px 8px 3px black;

    /* this is used for the resize cursor area */
    margin: 10px;
}

.window-frame.tiled {
    border-radius: 0;
}

.window-frame:backdrop {
    box-shadow: 0 2px 5px 1px black;
}