To export the modularStyles 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:
* modularStyles [ResourceLoader|hidden|default|type=styles] | ms-Tardis.css | ms-TableGrid.css | ms-Mainpage.css | ms-Namespace.css | ms-Navbox.css | ms-TardisAdmin.css | ms-Themes.css | ms-Transmat.css | ms-Forum.css | ms-UserWarning.css | ms-Familytree.css | ms-Carousel.css
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.