GTK 4.x

gtk.css

$XDG_CONFIG/gtk-4.0/gtk.css

window {
    border-radius: 0;
}

headerbar {
    min-height: 0;
    padding: 0 2px;
}

headerbar button {
    min-width: 16px;
}

headerbar menubutton {
    margin: 0;
}

headerbar menubutton button {
    margin: 4px;
}

headerbar .start,
headerbar .end {
    border-spacing: 0;
}

headerbar .start button,
headerbar .end button {
    margin: 2px;
}

headerbar stackswitcher button  {
    margin: 2px 0;
}

headerbar windowcontrols button {
    min-width: 22px;
    min-height: 22px;
}

.titlebar {
    min-height: 0;
}

Fix font hinting

$XDG_CONFIG/gtk-4.0/settings.ini

[Settings]
gtk-hint-font-metrics=1
gtk-font-rendering=manual

Fix incorrect (libadwaita?) theming

Install desktop portal backend, e.g. xdg-desktop-portal-gtk.
Set dconf /org/gnome/desktop/interface/color-scheme value to prerer-dark ([3] will produce errors and render transparent background without this (!)) .
Set empty GTK_THEME environment variable.

Notes:

The /org/gnome/desktop/interface/gtk-theme dconf value is expected to be set to Arc-Dark and is shared with other GTK versions.
If GTK_THEME is set to Arc-Dark and [2] is skipped, some applications (e.g. gnome-caluclator, baobab) do render correctly, but some (e.g. celluloid)- don't.