diff --git a/src/_h5ai/css/inc/preview-img.less b/src/_h5ai/css/inc/preview-img.less
index 6946de74..6d791b85 100644
--- a/src/_h5ai/css/inc/preview-img.less
+++ b/src/_h5ai/css/inc/preview-img.less
@@ -9,6 +9,7 @@
 	z-index: 100;
 
 	background-color: rgba(0,0,0,0.5);
+	.transition(background-color 0.3s ease-in-out);
 
 	text-align: center;
 }