Issue #3479019: Missing CSS specificity and wrong separator color in Blackstone

Also removed properties that are not changed or overriden

css/xnavigation.css

@@ -28,9 +28,7 @@
   color: #4E4943;
 }
 
-#admin-toolbar .admin-toolbar__content .toolbar-block:nth-last-child(n+2)::after {
-  margin-top: var(--admin-toolbar-space-8);
-  content: "";
+#admin-toolbar .xnavigation-blackstone .toolbar-block:nth-last-child(n+2)::after {
   border-top: 2px solid #43352C;
 }
 
@@ -80,9 +78,7 @@
   color: #74563B;
 }
 
-#admin-toolbar .admin-toolbar__content .toolbar-block:nth-last-child(n+2)::after {
-  margin-top: var(--admin-toolbar-space-8);
-  content: "";
+#admin-toolbar .xnavigation-slate .toolbar-block:nth-last-child(n+2)::after {
   border-top: 2px solid #2D4459;
 }