Change the window border size

To change the size edit the file ~.config/gtk-3.0/gtk.css.

.header-bar.default-decoration {
    padding-top: 3px;
    padding-bottom: 3px;
    font-size: 0.8em;
}

.header-bar.default-decoration .button.titlebutton {
    padding: 0px;
}

window.ssd headerbar.titlebar {
    padding-top: 0px;
    padding-bottom: 0px;
    min-height: 0;
}

window.ssd headerbar.titlebar button.titlebutton {
    padding: 5px;
    min-height: 0;
    min-width: 0;
}