Linux
Other
Edit ~/.config/gtk-3.0/gtk.css and log out / in.
.header-bar {
padding-top: 2.5px;
padding-bottom: 2.5px;
}
.header-bar .button {
padding-top: 4px;
padding-bottom: 4px;
}
.header-bar.default-decoration {
padding-top: 2px;
padding-bottom: 2px;
}
.header-bar.default-decoration .button.titlebutton {
padding-top: 3px;
padding-bottom: 3px;
}