add feature to change default theme of riot-web via its config file

roles/matrix-riot-web/defaults/main.yml

@@ -78,7 +78,9 @@ matrix_riot_web_themes_repository_url: https://github.com/aaronraimist/riot-web-
 
 # Controls the `settingsDefault.custom_themes` setting of the riot-web configuration.
 # You can use this setting to define custom themes.
-#
+matrix_riot_web_change_default_theme: false
+matrix_riot_web_default_theme: ""
+
 # Also, look at `matrix_riot_web_themes_enabled` for a way to pull in a bunch of custom themes automatically.
 # If you define your own themes here and set `matrix_riot_web_themes_enabled: true`, your themes will be preserved as well.
 #

roles/matrix-riot-web/templates/config.json.j2

@@ -11,6 +11,9 @@
 	"settingDefaults": {
 		"custom_themes": {{ matrix_riot_web_settingDefaults_custom_themes|to_json }}
 	},
+	{% if matrix_riot_web_change_default_theme %}
+		"default_theme": {{ matrix_riot_web_default_theme|string|to_json }},
+	{% endif %}
 	"permalinkPrefix": {{ matrix_riot_web_permalinkPrefix|string|to_json }},
 	"disable_custom_urls": {{ matrix_riot_web_disable_custom_urls|to_json }},
 	"disable_guests": {{ matrix_riot_web_disable_guests|to_json }},