To start, just create the style you want, name it, then copy the style but in the name add "
:focus" and that's all. After creating the CSS, then create the HTML form, link the CSS file, and add to the form whatever fields you want and assign them the style you just made. That's all, nothing else is needed.
To see it in action, download the fully editable example file.
That's all for now.
Regards from Argentina.