Feature: add more vector layers from maptiler; add some fixes to theme setting handler

assets/layers/usersettings/usersettings.json

@@ -24,7 +24,7 @@
     "_mastodon_candidate_a=(feat => {const e = document.createElement('div');e.innerHTML = feat.properties._d;return Array.from(e.getElementsByTagName(\"a\")).filter(a => a.href.match(/mastodon|en.osm.town/) !== null)[0]?.href   }) (feat) ",
     "_mastodon_link=(feat => {const e = document.createElement('div');e.innerHTML = feat.properties._d;return Array.from(e.getElementsByTagName(\"a\")).filter(a => a.getAttribute(\"rel\")?.indexOf('me') >= 0)[0]?.href})(feat) ",
     "_mastodon_candidate=feat.properties._mastodon_candidate_md ?? feat.properties._mastodon_candidate_a",
-    "__current_background='some value to let the validation run through'"
+    "__current_background='initial_value'"
   ],
   "tagRenderings": [
     {
@@ -104,11 +104,25 @@
         "*": "{logout()}"
       }
     },
+    {
+      "id": "background-layer-readonly",
+      "condition": {
+        "and": [
+          "_theme:backgroundLayer~*",
+          "mapcomplete-preferred-background-layer~*",
+          "_theme:backgroundLayer!:={mapcomplete-preferred-background-layer}"
+        ]
+      },
+      "render": {
+        "en": "This thematic map has a predefined background layer set. Your default theme setting does not apply"
+      }
+    },
     {
       "id": "background-layer",
       "question": {
         "en": "What background layer should be shown by default?"
       },
+      "condition": "_theme:backgroundLayer=",
       "mappings": [
         {
           "if": "mapcomplete-preferred-background-layer=",
@@ -140,7 +154,11 @@
             "en": "Use the current background layer (<span class='code'>{__current_background}</span>) as default background"
           },
           "hideInAnswer": {
-            "or": ["__current_background=","__current_background=osm"]
+            "or": [
+              "__current_background=",
+              "__current_background=osm",
+              "__current_background=initial_value"
+            ]
           }
         },
         {