How to modify the HTML, CSS and add JavaScript?

I want to create an interactive story, but one that has more than choices.
1) I want this story to have modals that pop up and display additional information after clicking on text, surveys, multiple choice questions, Internet links, and polls.
2) I want to be able to modify the HMTL/CSS enough to add style to any page, and create a uniquely looking story templates.

Is this possible? If so, how do I go about doing that?
If not, do I have any other options?

Thanks!
FJ Contreras