As I was working on cleaning up a few isses for the Javascript Tools module, I stumbled upon one issue dealing with overriding CSS styles. In this case, JSCalendar was adding it's styles after a theme's styles were being loaded. Obviously, from a CSS standpoint, it would be impossible for the theme to override and change CSS styles in JSCalendar. This needed to be fixed.
After some digging, I found this comment that explains this problem in more detail and outlines the correct procedure for modules and themes to add CSS files.
To summarize Tom:
<?php
// Modules should do:
drupal_set_html_head(theme('stylesheet_import', base_path() . drupal_get_path('module', 'mymodule') .'/mymodule.css'));
?>
<?php
// user-created PHP pages/blocks/comments/etc. should do:
theme_add_style('misc/local/my_custom_style.css');
// (theme_add_style is also used internally for the style.css files)
?>
<?php
// themes should do:
$output .= theme('stylesheet_import', base_path() . path_to_theme() . '/extra_stylesheet.css');
/* note that this should be BEFORE theme_get_styles, which will reference
the style.css file and anything added by the user... themers will probably also
want it located after drupal_get_html_head, which will bring in any module-specific CSS
*/
?>
