gtk.css

  background-color: transparent;
  background-image: -gtk-scaled(url("../assets/titlebutton-maximize-maximized-active.png"), url("../assets/titlebutton-maximize-maximized-active@2.png")); }

.maximized headerbar button.titlebutton.maximize:backdrop, .maximized
.titlebar button.titlebutton.maximize:backdrop {
  border-color: transparent;
  background-color: transparent;
  background-image: -gtk-scaled(url("../assets/titlebutton-maximize-maximized-backdrop.png"), url("../assets/titlebutton-maximize-maximized-backdrop@2.png")); }

headerbar.selection-mode button.titlebutton,
.titlebar.selection-mode button.titlebutton {
  text-shadow: none; }
  headerbar.selection-mode button.titlebutton:backdrop,
  .titlebar.selection-mode button.titlebutton:backdrop {
    -gtk-icon-shadow: none; }