Gnome CSS customizations - mainly narrower titlebar

gtk.css

/*
Gnome 3 - based on https://blog.samalik.com/make-your-gnome-title-bars-smaller/
https://securitronlinux.com/bejiitaswrath/how-to-make-the-title-bars-in-gnome-shell-much-thinner-than-the-default/
sizes tweaked to make the close button bigger on 20.04 (not tested on earlier versions)

Copy or symlink to ~/.config/gtk-3.0/gtk.css

To change titlebar font from Bold to normal (maybe there is CSS for this too?):
$ gsettings get org.gnome.desktop.wm.preferences titlebar-font
'Ubuntu Bold 12'
$ gsettings set org.gnome.desktop.wm.preferences titlebar-font 'Ubuntu 12'

Debugging:
    sudo apt install libgtk-3-dev
    gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
    GTK_DEBUG=interactive program-name
*/
.header-bar.default-decoration {
    padding-top: 1px;
    padding-bottom: 1px;
    font-size: 0.5em;
}
.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: 1px;
    padding-bottom: 1px;
    min-height: 0;
}
window.ssd headerbar.titlebar button.titlebutton {
    padding: 2px;
    min-height: 0;
    min-width: 0;
}

headerbar {
    min-height: 0px;
    padding-top: 2px;
    padding-bottom: 1px;
    padding-right: 1px;
    padding-left: 1px;
}

headerbar entry,
headerbar spinbutton,
headerbar button,
headerbar separator {
    margin-top: 0;
    margin-bottom: 0;
}

headerbar button {
    border: none;
    min-height: 16px;
    padding-top: 2px;
    padding-bottom: 2px;
}

headerbar stack box entry {
    min-height: 20px;
}

headerbar button.titlebutton.close {
    padding: 2px;
}

.titlebar,
.titlebar label.title {
    min-height: 0;
    font-weight: normal;
}

/* short tilix tabs */
notebook.tilix-background tab {
    padding: 2px;
    min-width: 0;
}
notebook.tilix-background tab label {
    font-weight: normal;
}
notebook.tilix-background * {
    padding: 0;
    margin: 0;
    min-height: 18px;
}