Dynamic Webpage creation with HTML and CSS

I have a puzzle , I want to create a an application which can allow users to create their own HTML forms the way they want it and add some styles if they want to . Which will serve as a custom template Now after having allows them to do this , I want to either save their HTML or code in the database or to a file and then read it when they want to use that custom HTML

What I need is expect advice on best practices about a situation like this 1. I am using Nodejs /Express with ejs and MySQL as the backend 2. My plan is to allow them to enter raw HTML and save it to the database and read it when they want to use that customer HTML , but how would external CSS be added if they have one 3. I also research and found Jquery form builder which will allow them to drag and drop and create they forms which I can get the HTML from it 4. Another problem is how will I allow users to apply their custom CSS to an existing page ? Please does anyone has experience on how to deal with a situation like this Any ideas would be welcomed which will allow users to creat their customer page and add their custom CSS if they do not want to use the default one provided