The appearance of windows in Gnome can be tweaked by applying custom CSS. Here is a hack for slimming down the height of application title bars.

Before

Gnome title bars are fairly chunky when you compare them to what you get in other environments such as Cinnamon, Unity or macOS.

Normal gnome title bar

After

With a simple hack you can reduce the size and intrusiveness of the title bar:

Slim gnome title bar

The Hack

Create the following CSS file:

# Gnome 3 - based on https://blog.samalik.com/make-your-gnome-title-bars-smaller/
.header-bar.default-decoration {
    padding-top: 3px;
    padding-bottom: 3px;
    font-size: 0.8em;
}
.header-bar.default-decoration .button.titlebutton {
    padding: 0px;
}


# --------------------------------------------------


# Gnome 3.20+ - based on https://blog.samalik.com/make-your-gnome-title-bar-smaller-fedora-24-update/
window.ssd headerbar.titlebar {
    padding-top: 4px;
    padding-bottom: 4px;
    min-height: 0;
}
window.ssd headerbar.titlebar button.titlebutton {
    padding: 0px;
    min-height: 0;
    min-width: 0;
}

Once you have created the file you will need to logout and back in for the change to take affect.

View gist.