add flip card animation

themes/common.css

@@ -170,14 +170,15 @@
     padding: 15px;
     margin: 10px;
     flex: 1 0 calc(30% - 5px);
-    transition: opacity 0.1s ease-in-out;
+    transform: rotateY(0deg);
-    opacity: 1;
+    transition: transform 0.1s;
+    transform-style: preserve-3d;
 }
 .welcome-card.show {
-    opacity: 1;
+    transform: rotateY(0deg);
 }
 .welcome-card.hide {
-    opacity: 0;
+    transform: rotateY(90deg);
 }
 .welcome-title {
     font-size: 40px;