There’s been many a time where I’ve created the perfect CSS in a static site design only for it to be pulled to pieces by Drupal’s system styles.
The solution is
hook_css_alter(&$css) which can reside in your template.php file:
To find out which stylesheets need to be removed, use web-developer tools / firebug and check which css file the styles are coming from.
Note: this might be impossible if you have CSS aggregation enbaled