Simplify styles for the menu button

Using `currentcolor` and `color: inherit` is IMHO more expressive (we're
saying we want to use the same color as the text) and makes it easier to
customize these colors in other CONSUL installations. We also remove
duplication as we can use the same styles for both the admin and the
public layouts.

app/views/layouts/_header.html.erb

@@ -16,7 +16,7 @@
     <div class="top-bar">
 
       <span data-responsive-toggle="responsive-menu" data-hide-for="medium" class="float-right">
-        <button type="button" class="menu-icon dark" data-toggle></button>
+        <button type="button" class="menu-icon" data-toggle></button>
         <%= t("application.menu") %>
       </span>