File at .config/gtk-3.0/gtk.css
headerbar entry,
headerbar spinbutton,
headerbar button,
headerbar separator {
margin-top: 2px; /* same as headerbar side padding for nicer proportions */
margin-bottom: 2px;
}
.default-decoration {
min-height: 0; /* let the entry and button drive the titlebar size */
padding: 2px
}
.default-decoration .titlebutton {
min-height: 16px; /* tweak these two props to reduce button size */
min-width: 16px;
}
window.ssd headerbar.titlebar {
border: none;
background-image: linear-gradient(to bottom,
shade(@theme_bg_color, 1.05),
shade(@theme_bg_color, 0.99));
box-shadow: inset 0 1px shade(@theme_bg_color, 1.4);
}