Improve sass styles

layouts/partials/docs/menu-filetree.html

@@ -8,7 +8,7 @@
 {{ define "book-section" }} <!-- Single section of menu (recursive) -->
   <ul>
     {{ range .Section.Sections }}
-    <li {{- if .Params.bookrootsection}} class="section" {{ end }}>
+    <li {{- if .Params.bookflatsection}} class="flat" {{ end }}>
       {{- if .Content -}}
         {{ template "book-page-link" (dict "Page" . "CurrentPage" $.CurrentPage) }}
       {{- else -}}

layouts/partials/docs/mobile-header.html

@@ -0,0 +1,4 @@
+<label for="menu-control">
+  <img src="/svg/menu.svg" />
+</label>
+<strong>{{- template "title" . }}</strong>