expandable search icon

When I put a search item in the header or a menu, the search field remains visible all the time. Is there any css I can add to make the search icon expand to include the search field only when clicked, as seen on the “Marketer” GP site?

https://gpsites.co/marketer/

I installed that site, and looked in widgets, elements, and plugins, but can't figure how to recreate it on other headers or menus.