How to costumize the input fields but only in the page /user? I want to create a particular style with css for input style in the field of login, register and request new password.
The css if i see it width firebug is only one and i must adding a type only for the form of the /user page, but not for all page that containes form.
Thank you for help!
Check out the classes added to the opening body tag in the HTML. It will help you narrow down styles to just that page.

I know one of my D6 sites, there is a class in the body tag 'page-user' so you can use some CSS such as...

body.page-user .form-text {
  /* Add styling */
}


Inspect the HTML and style as necessary. :o)