Sometimes in life, greater is not always better. Here’s a config which I used to make the Gnome 3 title bar smaller.

Create a file in ~/.config/gtk-3.0/gtk.css:

headerbar.default-decoration {
    padding-top: 3px;
    padding-bottom: 3px;
    min-height: 0px;
    font-size: 0.7em;
}

headerbar.default-decoration button.titlebutton {
    padding: 0px;
    min-height: 0px;
}

Here’s how the title bar looks like before and after applying the custom css file:

Default title bar size

Here’s a smaller title bar:

Small title bar

All credits go to the folks from these links: