[DarkMode] Deprecate advanced setting (#202993)

docs/management/advanced-options.asciidoc

@@ -187,7 +187,7 @@ this setting stores part of the URL in your browser session to keep the URL
 short.
 
 [[theme-darkmode]]`theme:darkMode`::
-The UI theme that the {kib} UI should use.
+deprecated:[9.0.0] The UI theme that the {kib} UI should use.
 Set to `enabled` or `disabled` to enable or disable the dark theme.
 Set to `system` to have the {kib} UI theme follow the system theme.
 You must refresh the page to apply the setting.

packages/core/ui-settings/core-ui-settings-server-internal/src/settings/theme.ts

@@ -65,6 +65,12 @@ export const getThemeSettings = (
           defaultMessage: `Sync with system`,
         }),
       },
+      deprecation: {
+        message: i18n.translate('core.ui_settings.params.darkModeDeprecation', {
+          defaultMessage: 'This setting is deprecated and will be removed in Kibana 10.0.',
+        }),
+        docLinksKey: 'generalSettings',
+      },
       requiresPageReload: true,
       schema: schema.oneOf([
         schema.literal('enabled'),