Answer is mixed. You can reuse the styling you have already done in html form but it is not as simple as copy-pasting the html form in drupal.
You need to use drupal form api for this purpose.
Drupal 6 form example: http://drupal.org/node/717734
Drupal 7 form example: http://drupal.org/node/717740
Drupal 6 field example:
$form['title'] = array(
'#type' => 'textfield',
'#title' => t('Page title'),
'#attributes' => array(
'class' => 'page-title',
),
);
Drupal 7 field example:
$form['title'] = array(
'#type' => 'textfield',
'#title' => t('Page title'),
'#attributes' => array(
'class' => array('page-title'),
),
);