Add theme switcher to header

Author: SebastianGrans

config/_default/menus.en.toml

@@ -35,6 +35,12 @@
   [main.params]
     action = "search"
 
+[[main]]
+  identifier = "changeappearance"
+  weight = 100
+  [main.params]
+    action = "appearance"
+
 # -- Footer Menu --
 # The footer menu is displayed at the bottom of the page, just before
 # the copyright notice. Configure as per the main menu above.