Am I right in understanding this as follows:
User will edit fields on a page such as "font-size:" and you want the page to dynamically change according to what they have entered?
If so I think the first thing you need to explore is jQuery. jQuery has methods that enable you to change the CSS (e.g. font-size or background-color) on the fly. You can use it to execute these methods on an particular event, e.g the click of a submit button.
So you'd have something like
//prepare the function that we want to be executed on submit button click
var makeChangesToPage = function() {
//record the user-entered value for the background color and store it as variable "bgcolor"
var bgcolor = $('#bgcolorfield').val();
//now change the CSS of the page (document.body) so that the background color is now equal to our stored bgcolor value
$(document.body).css('background-color', bgcolor);
}
//tell jQuery to kick off those changes whenever the button is actually clicked
$('#button_submit').click( makeChangesToPage() );
Where I've used $('#something') above, that is a jQuery selector to find any HTML elements where you have set the id as "something" e.g for the submit button we set its id like this when first creating the button on the page in our HTML:
<input type="submit" id="button_submit" value="Submit">
And then we can 'find' it to use in our JavaScript using the jQuery selector:
var myButton = $('#button_submit');
I've left most of the relevant and any other jQuery/CSS changes (after background-color) for you to work on but that should kick you off in the right direction.