clean up css for tablet, nav (#19477)

Author: kbhawkey

assets/sass/_base.sass

@@ -109,6 +109,7 @@ header
   box-shadow: 0 0 0 transparent
   transition: 0.3s
   text-align: center
+  overflow: hidden
 
 
 .logo
@@ -244,8 +245,7 @@ header
       background-color: white
 
 #mainNav
-  display: none
-  
+
   h5
     color: $blue
     font-weight: normal