Add transition to notification's close button background color

Packages/ohif-core/client/components/bootstrap/notification/notificationArea.styl

@@ -37,6 +37,7 @@
       position: absolute
       right: 0
       top: 0
+      transition(background-color 0.3s ease)
       width: 40px
 
       i
@@ -52,7 +53,7 @@
         width: 100%
 
       &:hover
-        background: rgba(0, 0, 0, 0.1)
+        background-color: rgba(0, 0, 0, 0.1)
 
         i
           transform(translateY(-50%) rotate(180deg))