Change titlebar height in Gnome - http://unix.stackexchange.com/questions/276951/how-to-change-the-titlebar-height-in-standard-gtk-apps-and-those-with-headerbars

_ubuntu_gnome_tweaks

Change titlebar height in Gnome 

http://unix.stackexchange.com/questions/276951/how-to-change-the-titlebar-height-in-standard-gtk-apps-and-those-with-headerbars

gtk.css

.header-bar.default-decoration {
        padding-top: 0px;
        padding-bottom: 0px;
    }

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

/* No line below the title bar */
.ssd .titlebar {
    border-width: 0;
    box-shadow: none;
}

window.ssd headerbar.titlebar {
    padding-top: 1px;
    padding-bottom: 1px;
    min-height: 0;
}

window.ssd headerbar.titlebar button.titlebutton {
    padding-top: 1px;
    padding-bottom: 1px;
    min-height: 0;
}
/* shrink headebars */
headerbar {
    min-height: 24px;
    padding-left: 2px; /* same as childrens vertical margins for nicer proportions */
    padding-right: 2px;
}

headerbar entry,
headerbar spinbutton,
headerbar button,
headerbar separator {
    margin-top: 2px; /* same as headerbar side padding for nicer proportions */
    margin-bottom: 2px;
}

/* shrink ssd titlebars */
.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;
}