Rainbow flag (#234)

* add rainbow

* fix

Author: simonebortolin

_layouts/default.html

@@ -46,6 +46,12 @@
       {% include icons/switch_color_scheme.html %}
     {% endif %}
     {% include icons/custom.html %}
+    <style>
+      .main-header, .site-header {
+        border-bottom: 1px solid #eeebee;
+        border-image: linear-gradient(90deg, white 0 7.69%, #ffa6b9 0 15.38%, #00d2ff 0 23.07%, #753000 0 30.76%, black 0 38.45%, #0000 0 46.14%, red 0 53.83%, orange 0 61.52%, yellow 0 76.9%, green 0 84.59%, blue 0 92.28%, indigo 0 100%) 10;
+     }
+    </style>
   </svg>
 
   <div class="side-bar">