Is it possible to add custom stylesheets while using a core theme? I'm trying to add a custom stylesheet although I'm using the Garland theme but so far I wasn't able to do it.

I've tried the following:

function mymodule_init(){
    $path = drupal_get_path('module', 'mymodule');
    drupal_add_js ( $path . '/extra/script.js' ); // script is loaded and works
    drupal_add_css( $path . '/extra/style.css' ); // css isn't.
}


I also have a form that I will display when accessing /myform, so I tried to add the CSS there:

function mymodule_formname($form_state){
    drupal_add_css( drupal_get_path('module', 'mymodule') . '/extra/style.css' );
    // $form definition here
    return $form;
}


No success. Finally also tried to include the CSS using the #attached form parameter but still no results. I flush the cache for each of the attempts just to be sure... 

Am I missing something? Shouldn't this be possible?
You can use context and context add assets to get the job done. If you are not familiar with context it's a way to set reaction blocks, theme HTML, theme pages, menus, etc based on conditions on menu items, paths, node types, taxonomy terms, etc. You can include a custom css file on your form with context add assets.