Update manifest.json

use latest property name for css themes extension.

Author: julkuh

src/main/js/bundles/mapapps-github-manager/manifest.json

@@ -18,7 +18,7 @@
     "editor": {
         "suggest": false
     },
-    "CSS-Themes-Extension": [
+    "cssThemesExtension": [
         {
             "name": "*",
             "files": [