Issue #3478945: Fix missing CSS files

css/xnavigation-blackstone.css

+/* Blackstone */
+
+#admin-toolbar .xnavigation-blackstone .toolbar-menu--level-2 {
+  box-shadow: none;
+}
+
+#admin-toolbar .xnavigation-blackstone .toolbar-button {
+  color: white;
+}
+
+#admin-toolbar .xnavigation-blackstone .toolbar-menu__link,
+#admin-toolbar .xnavigation-blackstone + nav#menu-footer.admin-toolbar__footer .toolbar-button {
+  color: #797167;
+}
+
+#admin-toolbar .xnavigation-blackstone,
+#admin-toolbar .xnavigation-blackstone .admin-toolbar__header,
+#admin-toolbar .xnavigation-blackstone .toolbar-button,
+#admin-toolbar .xnavigation-blackstone > nav,
+#admin-toolbar .xnavigation-blackstone + nav,
+#admin-toolbar .xnavigation-blackstone .toolbar-popover__wrapper,
+#admin-toolbar .xnavigation-blackstone .toolbar-menu__item--level-1,
+#admin-toolbar .xnavigation-blackstone .toolbar-menu--level-2,
+#admin-toolbar .xnavigation-blackstone + nav .toolbar-popover__wrapper,
+#admin-toolbar .xnavigation-blackstone + nav#menu-footer.admin-toolbar__footer .toolbar-button {
+  background: #1D1E1C;
+}
+
+#admin-toolbar .xnavigation-blackstone .toolbar-popover__header {
+  color: #4E4943;
+}
+
+#admin-toolbar .xnavigation-blackstone .toolbar-block:nth-last-child(n+2)::after {
+  border-top: 2px solid #43352C;
+}
+
+#admin-toolbar .xnavigation-blackstone button,
+#admin-toolbar .xnavigation-blackstone + nav .toolbar-popover__wrapper button {
+  background: #594739;
+}
+
+#admin-toolbar .xnavigation-blackstone + nav {
+  border-top-color: #43352C;
+}
+
+#admin-toolbar #menu-builder.xnavigation-blackstone,
+#admin-toolbar .xnavigation-blackstone + nav#menu-footer {
+  border-right-color: #43352C;
+}

css/xnavigation-slate.css

+/* Slate */
+
+#admin-toolbar .xnavigation-slate .toolbar-menu--level-2 {
+  box-shadow: none;
+}
+
+#admin-toolbar .xnavigation-slate .toolbar-button {
+  color: white;
+}
+
+#admin-toolbar .xnavigation-slate .toolbar-menu__link,
+#admin-toolbar .xnavigation-slate + nav#menu-footer.admin-toolbar__footer .toolbar-button {
+  color: #9498A1;
+}
+
+#admin-toolbar .xnavigation-slate,
+#admin-toolbar .xnavigation-slate .admin-toolbar__header,
+#admin-toolbar .xnavigation-slate .toolbar-button,
+#admin-toolbar .xnavigation-slate > nav,
+#admin-toolbar .xnavigation-slate + nav,
+#admin-toolbar .xnavigation-slate .toolbar-popover__wrapper,
+#admin-toolbar .xnavigation-slate .toolbar-menu__item--level-1,
+#admin-toolbar .xnavigation-slate .toolbar-menu--level-2,
+#admin-toolbar .xnavigation-slate + nav .toolbar-popover__wrapper,
+#admin-toolbar .xnavigation-slate + nav#menu-footer.admin-toolbar__footer .toolbar-button {
+  background: #122537;
+}
+
+#admin-toolbar .xnavigation-slate .toolbar-popover__header {
+  color: #74563B;
+}
+
+#admin-toolbar .xnavigation-slate .toolbar-block:nth-last-child(n+2)::after {
+  border-top: 2px solid #2D4459;
+}
+
+#admin-toolbar .xnavigation-slate button,
+#admin-toolbar .xnavigation-slate + nav .toolbar-popover__wrapper button {
+  background: #74563B;
+}
+
+#admin-toolbar .xnavigation-slate + nav {
+  border-top-color: #2D4459;
+}
+
+#admin-toolbar #menu-builder.xnavigation-slate,
+#admin-toolbar .xnavigation-slate + nav#menu-footer {
+  border-right-color: #2D4459;
+}