I added these custom js and css file:
skin/frontend/base/default/black/NewsletterPopup/js/main.jsskin/frontend/base/default/black/NewsletterPopup/css/main.js
Then I included them via my modules layout file
app/design/frontend/base/default/layout/black/newsletter_popup.phtml
<?xml version="1.0"?>
<layout version="0.1.0">
<default>
<reference name="head">
<action method="addJs">
<script>black/NewsletterPopup/js/main.js</script>
</action>
<action method="addCss">
<stylesheet>black/NewsletterPopup/css/main.css</stylesheet>
</action>
</reference>
</default>
<!-- ... -->
But a network analysis showed that the files could not be found and if I enter the link to the file directly into the URL then the "404 Sorry page not found" page shows up.
What am I doing wrong?