Adding option to brand across services where possible

roles/matrix/defaults/main.yml

@@ -1,2 +1,4 @@
 ---
 matrix_data_dir: "{{ sovereign_base_dir }}/matrix"
+# Element theme: "light" or "dark"
+element_theme: "light"

roles/matrix/templates/element-config.json.j2

@@ -5,7 +5,7 @@
       "server_name": "{{ matrix_domain }}"
     }
   },
-  "brand": "Element",
+  "brand": "{{ tenant_name }}",
   "integrations_ui_url": "https://scalar.vector.im/",
   "integrations_rest_url": "https://scalar.vector.im/api",
   "bug_report_endpoint_url": "",
@@ -13,7 +13,7 @@
   "show_labs_settings": false,
   "features": {},
   "default_federate": true,
-  "default_theme": "light",
+  "default_theme": "{{ element_theme | default('light') }}",
   "room_directory": {
     "servers": ["{{ matrix_domain }}"]
   },