fix flex-nav z-index

packages/rocketchat-theme/client/imports/base.less

@@ -1110,7 +1110,7 @@ label.required::after {
 	top: 0;
 	left: 0;
 	height: 100%;
-	z-index: 3;
+	z-index: 2;
 	overflow-y: auto;
 	overflow-x: hidden;
 	width: @rooms-box-width;
@@ -1370,7 +1370,7 @@ label.required::after {
 		top: 0;
 		left: 0;
 		width: 100%;
-		z-index: 2;
+		z-index: 1;
 		cursor: pointer;
 		min-height: @header-min-height;
 		height: @header-min-height;
@@ -1380,7 +1380,7 @@ label.required::after {
 		position: absolute;
 		top: 18px;
 		right: 8px;
-		z-index: 3;
+		z-index: 2;
 		cursor: pointer;
 	}
 
@@ -1393,7 +1393,7 @@ label.required::after {
 		text-align: right;
 		min-height: @footer-min-height;
 		height: @footer-min-height;
-		z-index: 3;
+		z-index: 1;
 
 		.logo {
 			display: block;