Add missing :focus styles

Double up :hover styles to also apply on :focus (particularly the
masthead button needs this, as otherwise it gives no visible indication
of being focused)

docs/assets/scss/_masthead.scss

@@ -39,7 +39,8 @@
     color: $bd-yellow;
     border-color: $bd-yellow;
 
-    &:hover {
+    &:hover,
+    &:focus {
       color: $bd-graphite;
       background-color: $bd-yellow;
       border-color: $bd-yellow;