Crie ou modifique ~/.config/gtk-3.0/gtk.css
.header-bar.default-decoration {
padding-top: 3px;
padding-bottom: 3px;
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);
}
.header-bar.default-decoration .button.titlebutton {
padding-top: 2px;
padding-bottom: 2px;
}
Em seguida, reinicie o shell do GNOME com o comando Alt + F2 e r
as.
Source : link