Hi, I'm trying to customize login form to tailor HTML to my design.

I added

Object::useCustomClass('MemberLoginForm', 'CustomLoginForm');

to mysite/_config.php.
Created CustomLoginForm.php in mysite/code:

class CustomLoginForm extends MemberLoginForm 
{

	public function __construct($controller, $name)
	{

			if(!$fields) {
				$label=singleton('Member')->fieldLabel(Member::config()->unique_identifier_field);
				$fields = new FieldList(
					new HiddenField("AuthenticationMethod", null, $this->authenticator_class, $this),
					// Regardless of what the unique identifer field is (usually 'Email'), it will be held in the
					// 'Email' value, below:
					new TextField("Email", $label, Session::get('SessionForms.MemberLoginForm.Email'), null, $this),
					new PasswordField("Password", _t('Member.PASSWORD', 'Password'))
				);
				if(Security::config()->autologin_enabled) {
					$fields->push(new CheckboxField(
						"Remember", 
						_t('Member.REMEMBERME', "Remember me next time?")
					));
				}
			}

	parent::__construct($controller, $name);

	}

}

And copied some stuff from parent class just to make sure it works.

I get

[Warning] Missing argument 1 for CustomLoginForm::__construct()

and I don't know what is going on...

I got some help on IRC channel. Thanks Zauberfish!

I don't know what caused the warning but here is the code I use to render a field using custom ss file.

class CustomLoginForm extends MemberLoginForm {

	public function __construct($controller, $name, $fields = null, $actions = null, $checkCurrentUser = true)
	{
		parent::__construct($controller, $name, $fields, $actions, $checkCurrentUser);
		foreach($this->Fields() as $field) {
			if (!$field->is_a('HiddenField')) {
				// skip hidden fields

				$field->setFieldHolderTemplate('CustomFrontEndField_holder');
			}
		}
	}
}

To help avoid others an hours worth of head scratching: make sure to set strong to true on useCustomClass in /_config.php.

Example code adds Register link to SilverStripe login form. Using first MemberProfilePage module's page in the system (replace as required).

mysite/_config.php

Object::useCustomClass('MemberLoginForm', 'LoginOrRegisterForm', true);

mysite/code/LoginOrRegisterForm.php

<?php

class LoginOrRegisterForm extends MemberLoginForm {
	public function __construct($controller, $name, $fields = null, $actions = null, $checkCurrentUser = true) {
		Debug::Show("LoginOrRegister");
		parent::__construct($controller, $name, $fields, $actions, $checkCurrentUser);
		$this->Actions()->FieldByName('action_dologin')->addExtraClass('button small');
		$this->Actions()->insertAfter(new LiteralField('Add', '<a href="' . MemberProfilePage::get()->First()->Link() . '" class="right button small">Register &gt;</a>'), "action_dologin");
	}
}