To export the darkMode 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:
* darkMode[ResourceLoader|dependencies=mediawiki.util,oojs-ui.styles.icons-accessibility|targets=desktop|type=general|default|hidden|skins=vector,monobook,timeless,vector-2022]|darkMode.js|darkMode.css
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.