Theme and nav improvements: lang switcher styling, dropdown, theme thumbnail

- Style language switcher dropdown to match site colours
- Move language switcher left of social icons
- Set theme thumbnail to logo PNG in blueprints
- Add Dutch language override and system config tweaks
- Various CSS fixes for dropdown alignment and width

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

themes/natascha/css/custom.css

@@ -81,6 +81,27 @@ body {
     box-shadow: 0 0 0 0.25rem rgba(255, 255, 255, 0.25);
 }
 
+#main-menu .nav-item.dropdown .nav-link {
+    line-height: 25px;
+    padding-top: 0;
+    padding-bottom: 0;
+}
+
+#main-menu .dropdown-menu {
+    background-color: #0081c4;
+    border-color: #fff;
+    min-width: 0;
+}
+
+#main-menu .dropdown-menu .dropdown-item {
+    color: #fff;
+}
+
+#main-menu .dropdown-menu .dropdown-item:hover {
+    background-color: #0081c4;
+    color: #293236;
+}
+
 img.social-icon {
     filter: invert(1);
 }