Hi PaoloBax,
1. The Sample app opens https://www.htmlelements.com/demos/dropdownlist/overview/. This is not a Fake form. This is a sample built with our web components.
2. Styling of the Web Components is done in smart.default.css. If you would like to put custom CSS, take it into account that such style could be already defined. In your case, to put custom Background of these Inputs, you should use something like that:

smart-numeric-text-box .smart-input,
smart-text-box .smart-input,
smart-password-text-box .smart-input{
  background:blue;
}

3. Hints and Labels are not yet supported. We will add support for these in future versions of our web components. I would suggest you to take a look at the demos and how we use the components.
4. The roadmap is available here: https://www.htmlelements.com/docs/roadmap/
5. In general, this is an active project and we make frequent updates with bug fixes and new features.
Best Regards,
B. Markov
SmartHTMLElements
https://www.htmlelements.com