Render user login form with custom theme I have written a custom module, inside which I would like to create a custom user login block with custom theme applied. Now my question is "Inside my custom module, how do I get the user login form?"
You can use drupal_get_form() to get the rendered form:

$form = drupal_get_form('user_login');


To override the default theme for the form have a look at Theming the contact form in Drupal 6 which goes through all the necessary steps very thoroughly.