Fixes #11468: Prevent default gradient on in Firefox for Android

dist/css/bootstrap.css

@@ -4475,6 +4475,7 @@ textarea.input-group-sm > .input-group-btn > .btn {
   margin-right: 15px;
   margin-bottom: 8px;
   background-color: transparent;
+  background-image: none;
   border: 1px solid transparent;
   border-radius: 4px;
 }

less/navbar.less

@@ -180,6 +180,7 @@
   padding: 9px 10px;
   .navbar-vertical-align(34px);
   background-color: transparent;
+  background-image: none; // Fix for at least Firefox on Android, per #11468
   border: 1px solid transparent;
   border-radius: @border-radius-base;