Fix transparent bg for expand sidebar button

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/framework/sidebar.scss

@@ -200,7 +200,7 @@
   top: 0;
   left: 0;
   font-size: 20px;
-  background: transparent;
+  background: #fff;
   height: 59px;
   text-align: center;
   line-height: 59px;