header and custom header

I’m still running my site on local Mamp. My question is about making the custom header, which I managed to use by inserting <div> into the header.php and editing it with css, working as well as the header that one can upload with the Wp Header feature in Dashboard.
The point is, I would like my custom header to work as flexible as the Wp one and respond dynamically to the changes whilst making the browser smaller/bigger and on various devices.