Override a stylesheet added by a Drupal 7 theme

gistfile1.php

/**
 * Implements hook_element_info_alter().
 */
function mymodule_element_info_alter(&$type) {
  // Rather than implementing hook_css_alter(), we'll add a custom pre render 
  // function for the styles elemement. This is to allow us to override 
  // seven_css_alter(), which always runs after any module's implementation.
  if (isset($type['styles']['#pre_render'])) {
    array_unshift($type['styles']['#pre_render'], 'mymodule_pre_render_styles');
  }
}

/**
 * Pre render the styles element.
 */
function mymodule_pre_render_styles($elements) {
  $css = &$elements['#items'];

  // Installs the jquery.ui themeroller theme to the theme.
  if (isset($css['misc/ui/jquery.ui.theme.css'])) {
    $css['misc/ui/jquery.ui.theme.css']['data'] = drupal_get_path('module', 'mymodule') . '/css/mymodule-jquery-ui-theme.css';
  }
  
  if (isset($css['misc/ui/jquery.ui.dialog.css'])) {
    unset($css['misc/ui/jquery.ui.dialog.css']);
  }
  
  if (isset($css['misc/ui/jquery.ui.tabs.css'])) {
    unset($css['misc/ui/jquery.ui.tabs.css']);
  }

  return $elements;
}

A Drupal 7 theme can implement hook_css_alter() to insert custom css files in place of those added by modules. Seven in core does this to add its own jQuery UI styles for example. But sometimes you just need to override the override...