[SITE CSS] fix some spacing issue at mobile layout

- bigger margin on the bottom of the footer
- invert font color of mobile menu button at photos pages
- swap li and a tags in mobile menu. bigger width menu not changed, because big click target is needed
- bigger margin in top of the headings h1 to h4

assets/css/templates/photo.css

@@ -14,6 +14,10 @@ a {
   nav ul.nav.main {
     color: #fff; } }
 
+@media (max-width: 543px) {
+  .navbar-toggler {
+    color: #fff; } }
+
 footer {
   background-color: #303030;
   color: #e6e6e6; }