Gnome theme tweaks stolen from https://www.reddit.com/r/unixporn/comments/1ax1rb0/gnome_libadwaita_tweaked_elementary_style/

gtk3.css

/* .config/gtk-3.0/gtk.css */

window.ssd headerbar.titlebar {
  min-height: 0;
  padding-top: 2px;
  padding-bottom: 2px;
}
window.ssd headerbar.titlebar button.titlebutton {
  min-height: 0;
  min-width: 0;
  padding: 1px;
}
headerbar {
  border-radius: 5px 5px 0 0;
}
headerbar .title{
  border-radius: 5px 5px 0 0;
}
.csd .titlebar {
  border:solid 1px rgba(0, 0, 0, 0.3);
  border-radius: 5px 5px 0 0;
  box-shadow: 0 20px 30px rgba(0, 0, 0, 0.5);
}
decoration, window, window.background, window.titlebar {
  border-radius: 5px 5px 0 0;
}
.window-frame {
  border-radius: 5px 5px 0 0;
}

gtk4.css

/* .config/gtk-4.0/gtk.css */

window {
  border: solid 1px rgba(0, 0, 0, 0.3);
  border-radius:5px;
  box-shadow: 0 20px 30px rgba(0, 0, 0, 0.5);
} 
.nautilus-window .navigation-sidebar > .sidebar-row:nth-child(2)
{
  margin-top: -2px;
  min-height:0;
  font-size: 0;
  -gtk-icon-size: 0;
}
.nautilus-window .sidebar-pane .top-bar{
  min-height:0;
  max-height:0;
  margin-top: -12px;
  overflow:hidden;
  font-size: 0;
  -gtk-icon-size: 0;
}
.nautilus-window .sidebar-pane .top-bar *{
  min-height:0;
  max-height:0;
  margin-top: -2px;
  overflow:hidden;
  font-size: 0;
  -gtk-icon-size: 0;
}