How can i add a customization/Set Up page to a JS website

I want to add a page where users of my template can easily change the name of the website, logo etc to their taste by new typing the name into a text box or uploading it.

I want the website to be easily customized like wordpress.

I am doing this by adding the external scripts for customization to the various pages, but its not working. The html pages are no responding to the scripts. I am getting error = Uncaught TypeError: Cannot set properties of null (setting ‘src’).

I will appreciate every feed back. Thank you.