Minor Sass consistency changes. (#24677)

* use `background-color` instead of the shorthand
* use `outline: 0` consistently
* fix transform order
* remove quotes from `SFMono-Regular`

scss/_dropdown.scss

@@ -89,7 +89,7 @@
   color: $dropdown-link-color;
   text-align: inherit; // For `<button>`s
   white-space: nowrap; // prevent links from randomly breaking onto new lines
-  background: none; // For `<button>`s
+  background-color: transparent; // For `<button>`s
   border: 0; // For `<button>`s
 
   @include hover-focus {