GTK 3.x

Example settings

$XDG_CONFIG/gtk-3.0/settings.ini

[Settings]
gtk-theme-name=Arc-Dark
gtk-fallback-icon-theme=gnome
gtk-application-prefer-dark-theme=true
gtk-font-name=Verdana 10
gtk-toolbar-style=GTK_TOOLBAR_ICONS

gtk-xft-antialias=1
gtk-xft-hinting=1
gtk-xft-hintstyle=hintfull
gtk-xft-rgba=none

Environmental variables

GTK_THEME=Arc-Dark

gtk.css

Unbind F10 key

$XDG_CONFIG/gtk-3.0/gtk.css

@binding-set NoKeyboardNavigation {
    unbind "<shift>F10"
}

* {
    gtk-key-bindings: NoKeyboardNavigation
}

Expand & collapse single row in tree view with the arrow keys

$XDG_CONFIG/gtk-3.0/gtk.css

@binding-set TreeNavigation {
    bind "Left" {
        "select-cursor-parent" ()
        "expand-collapse-cursor-row" (0,0,0)
    };
    bind "Right" {
        "expand-collapse-cursor-row" (0,1,0)
    };
}

treeview {
    -gtk-key-bindings: TreeNavigation;
}

Compact header bar

$XDG_CONFIG/gtk-3.0/gtk.css

.window-frame {
    margin: 0 0 4px 0;
    box-shadow: none;
    background-color: #4c5153;
}

.header-bar,
.titlebar
{
    padding: 2px;
    border-radius: 0;
    box-shadow: none;
}

.header-bar .title,
.titlebar .title
{
    font-size: 80%;
}

.header-bar .subtitle {
    font-size: 60%;
}

.header-bar .button.image-button {
    padding: 5px;
}

.header-bar .titlebutton {
    padding: 5px;
}

Compact tabs

$XDG_CONFIG/gtk-3.0/gtk.css

.notebook {
    -GtkNotebook-tab-overlap: 0;
    -GtkNotebook-initial-gap: 0;
}

.notebook tab {
    padding: 5px;
}

GTK 3.20

$XDG_CONFIG/gtk-3.0/gtk.css

window > decoration,
window.csd,
headerbar {
    border-radius: 0;
}

headerbar {
    min-height: 16px;
    padding: 0 2px 0 2px;
}

headerbar entry,
headerbar spinbutton,
headerbar button,
headerbar separator {
    margin: 2px 0 2px 0;
}

headerbar.default-decoration {
    min-height: 0;
}

headerbar .title,
titlebar .title {
    font-size: 80%;
}

headerbar .subtitle {
    font-size: 65%;
}

headerbar viewswitcher button stack box {
    padding: 0 8px;
}

levelbar {
    padding: 1px;
}

paned {
    min-height: 0;
    margin: 0;
    padding: 0;
    border: 0;
}

button,
entry,
popover,
level-bar,
header-bar,
titlebar,
trough,
decoration,
GtkSwitch.slider {
    border-radius: 1px;
}

@binding-set TreeNavigation {
    bind "Left" {
        "select-cursor-parent" ()
        "expand-collapse-cursor-row" (0,0,0)
    };
    bind "Right" {
        "expand-collapse-cursor-row" (0,1,0)
    };
}

treeview {
    -gtk-key-bindings: TreeNavigation;
}

.xfce4-panel {
   font-family: "Verdana";
   font-size: 12px;
   font-weight: normal;
}

Qt5 style (Forky+)

Install qgnomeplatform-qt5 and qt-style-kvantum (for dark theme variant) and set QT_QPA_PLATFORMTHEME environment variable to gnome.

Another alternative to kvantum is to install libxapp-gtk3-module (requires session restart) and xdg-desktop-portal-gtk (?). This even applies to stubborn applications (e.g. Texmaker) but kvantum gives closer appearance to Gtk theme.