I think so? Well this is what I've done for the username, password etc;I like it, it's nice.
If it could be improved, try a different alignment. For example:
[Username-box] -space- [email-box] <br/>
[password1-box] -space- [password2-box]
^ If you get what I mean?
<form action="register.php" method="POST">
<div class="modal-header" style="background-color: rgba(85, 85, 85, 0.63);border-top-left-radius: 5px;border-top-right-radius: 5px;">
<h2>Register</h2>
<p><?php echo $register_msg; ?></p>
</div>
<div class="modal-body" style="background-color: rgba(85, 85, 85, 0.63);">
<table>
<tr>
<td><input type="text" name="username" style="height: 25px;" placeholder="Username"></td>
</tr>
<tr>
<td><input type="password" name="password" style="height: 25px;" placeholder="Password"></td>
</tr>
<tr>
<td><input type="password" name="password2" style="height: 25px;" placeholder="Confirm Password"></td>
</tr>
<tr>
<td><input type="text" name="email" style="height: 25px;" placeholder="Email"></td>
</tr>
</table>
</div>