To install this plugin you need to download it from the button link below and then install the plugin from the zip file you’ve downloaded.
Once you’ve installed and activated the plugin, you can then navigate to Plugins -> Plugin Editor in your WordPress Dashboard and on the top right you need to select to edit the Overlay Customizations Plugin and then click Select.
Then, on the right you will see a folder /custom/… Open that and there will be the 3 different files to edit, depending which code you’re wanting to add to your theme.
Select the relevant file according to the type of code you’re wanting to add.
— Edit functions.php to add your own PHP functions.
— Edit style.css to add your own custom CSS code.