Gadget export
To export the dark-mode-toggle gadget, click on "Download" button, save the downloaded file, go to Special:Import on destination wiki and upload it. Then add the following to MediaWiki:Gadgets-definition page:
* dark-mode-toggle [ResourceLoader |dependencies=mediawiki.util, mediawiki.api, mediawiki.Uri, mediawiki.storage |peers=dark-mode-toggle-pagestyles] |dark-mode-toggle.js
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.