theme_add_style(drupal_get_path('module', 'mymodule')).'/ mymodule.css');
This is often called in hook_menu() so as to get in as early as possible.
Themes almost always need to override those css files. The way it is done is to go to the mymodule directory and edit mymodule.css.
The problem here is when one upgrades, they have to be very careful not to overwrite the changes. What is worse, if one is upgrading from cvs directly, then there could be conflicts resulting in invalid data in the file.
As far as I know, theme_add_style() is not and never has been a themable function. In 4.7, it is only once invoked in its themable form, which is obviously as a mistake. And, ironically that's inside theme_maintenance_page(), which is unthemable by definition. The proper function to use in 4.7 is theme('stylesheet_import'). In spite of this, I don't see why you'd need to edit module .css files. Due to the CSS cascade, you can just provide more specific CSS rules in your theme to override them. Depending on the order the theme's stylesheet is added in, you either need to use the same specificity, or one level more, for example prefixing the CSS selector with "html". http://www.w3.org/TR/CSS1#the-cascade In 5.0, this problem has been solved, as the theme_add_style()/theme ('stylesheet_import') functions have been replaced by drupal_add_css (), and themes get a nice structured array of all stylesheets back in $css, which they can hack up as much as they want. You can then do drupal_get_css($css) to render this structured array into the right set of HTML headers. Steven Wittens