MC-4116: Update slider to use new background images logic

- fix poster overlay on stage when rendered in Block

Author: anthoula

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/slide/_poster.less

@@ -24,3 +24,24 @@
         }
     }
 }
+
+//
+//  Poster appearance styles rendered through Block
+//  ______________________________________________________
+
+
+[data-appearance='poster'] {
+    .pagebuilder-overlay {
+        align-items: center;
+        display: flex;
+        justify-content: center;
+
+        .pagebuilder-poster-content {
+            text-align: center;
+        }
+
+        .pagebuilder-collage-content {
+            padding: 30px;
+        }
+    }
+}