Use Nodejs with already made HTML/CSS

Question

I just started learning Nodejs and I want to use it to handle server side code on my site such as a Log In system

I have a login.html file with the following code, and a CSS file referenced to style this page. Here's the HTML form:

        <div id="login">
            <form id="loginForm" name="loginForm" action="" method="post">
                <input id="username" type="text" placeholder="Username" required><br>
                <input id="password" type="password" placeholder="Password" required><br>
                <input id="enterBtn" type="submit" name="enter" value="LOGIN" onClick="contactForm(this.form)">
            </form>
        </div>

Everywhere I've looked so far it seems that I have to use .ejs files in order to communicate with my server.js script.

But when I try copy/pasting my login.html code into a login.ejs file, but:

1. my css doesn't work anymore
2. when I ahref my login.ejs page from an html page, instead of opening the page, it opens the login.ejs code in the browser instead. Any suggestions? Thank you!