How do I override a module's stylesheet in my theme?

<rant>
This is still something that the Drupal core should be doing. The situation as it is now--even in version 5.x--is a major pain for theme developers and site-maintainers and, frankly, enforces poor css-coding practices. For example, if it's necessary/desirable to override the event module's default css in a theme, the only option available by default is to override event.css in the theme's stylesheet (i.e. since the module stylesheets should under no circumstances be edited directly--if the modules are ever updated, any such changes would be lost).

This results in a great number of duplicate css declarations being present in any theme where the various module defaults are, for one reason or another, unsuitable.

Furthermore, the default output of the css block in the document head is quite clueless--a separate style element for every single stylesheet?! What's wrong doing it this way:

<style type="text/css" media="all">
  @import "/lorem.css";
  @import "/ipsum.css";
  @import "/dolor.css";
</style>

... instead?

It could be argued that the default way provides a widely-compatible way of specifying the stylesheet's media, but since the default values are all "all", I'd think it would be better if the default were formatted as above and that theme authors could re-output them differently in the (less common) case where different media values are required.

I know that these defaults can be overridden--that's what this discussion is about--but as defaults, they're quite mediocre.
</rant>

In any case, anyone that's just read through my rant should be rewarded, so here's a new function that can be used to force Drupal to use theme-specific stylesheets when they are available. It looks through all of the available stylesheets and checks to see if there is a similarly-named file available in the current theme directory. If there is, that file is used instead. So, for example, a stock Drupal site may have a call to the file "/sites/default/modules/event/event.css", but this script replaces that call with a call to "/sites/default/themes/{theme name}/styles/event/event.css" The function uses Drupal 5 API calls:

/**
 *	This function replaces default Drupal stylesheets with theme-specific stylesheets IF they exist:
 */
function use_local_css_files($css,$media='all') {
	// Determine the path to the theme's styles directory from the web root:
	$webroot_theme_styles_directory = path_to_theme() . '/styles/';
	// Determine the path to the theme's styles directory from the absolute root:
	$root_theme_styles_directory = dirname($_SERVER['SCRIPT_FILENAME']) . '/' . $webroot_theme_styles_directory;
	// Make sure we start from a blank slate:
	$new_css_array = array();
	$new_default_css_array = array();
	$new_theme_css_array = array();
	// Loop through the incoming array of module css files--if there are any!:
	if (count($css[$media]['module']) > 0) {
		foreach($css[$media]['module'] as $key => $value) {
			// First, we have to strip out 'modules/' or 'sites/default/modules/':
			/*
				This regex says: replace any sub-strings containing:
					*- zero or more occurrences of "sites/default/" and
					*- one occurence of "modules/"
				...or in other words, any sub-strings containing:
					"sites/default/modules/" or "modules/"
				...with an empty string ('').
			*/
			$stylesheet_location = preg_replace('/(sites\/default\/)*(modules\/)/', '', $key);	
			// Put together the path from root (/) to the current stylesheet:
			$root_theme_stylesheet_path = $root_theme_styles_directory . $stylesheet_location;
			// Test to see if the local version exists:
			if (file_exists($root_theme_stylesheet_path)) { // When it does:
				// Build the path relative to the current theme:
				$theme_stylesheet_path = $webroot_theme_styles_directory . $stylesheet_location;
				// Add it to the array of THEME stylesheets:
				$new_theme_css_array[$theme_stylesheet_path] = $value;
			} else { // Otherwise:
				// Add the default stylesheet to the array of module stylesheets:
				$new_default_css_array[$key] = $value;
			}
		}
	}
	// Put everything together in the array format Drupal likes:
	$new_css_array[$media]['module'] = $new_default_css_array;
	$new_css_array[$media]['theme'] = array_merge($new_theme_css_array, $css[$media]['theme']);
	// Process everything we've got so far with drupal's main stylesheet function:
	$output = drupal_get_css($new_css_array);
	// Return the css calls:
	return $output;
}

Add this function to your template.php file, and call it in page.tpl.php. The function processes one media type at a time, so you may need to run it more than once (this is probably not ideal, but I only discovered how media types were stored in the css array after I wrote the basic function...this was the easiest way to support other media types...) Sample code follows:

	// Add a print stylesheet to the theme:
	$css = drupal_add_css(path_to_theme() . '/styles/print.css', 'theme', 'print', true);
	// Output all the 'media="all"' stylesheets:
	echo use_local_css_files($css);
	// Output all the 'media="print"' stylesheets:
	echo use_local_css_files($css, 'print');

Tested and apparently working with Drupal 5.1.