See the step-by-step instruction at below:
Open the functions.php file of your Atmosphere Pro theme and add the following snippets at end of the file. Before editing the file you will keep a backup of your child theme.
Creating a JS file “toggle-search.js” and put into the js folder. Here is the code of this file:
Add this simple PHP script in your functions.php file. It will load the toggle-search.js file on your site.
This is the last step. Open the style.css file and add the following CSSs at end of the file: