How to costumize the input fields but only in the page /user?
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)