The default theming for any form element can be easily override in Drupal. There are two choices in the place of using the override function. One is in the template file, other is in modules. I am going to demonstrate the theming in module using Drupal 6. Using template file or Drupal 7 do not have major different. I will show how to use them in the end of this page.
There are 2 different theming function for radios. One is, single radio button theming, other is group of radio button theming. The following theming functions are used by default for theming the above.