Recommended not to edit ".js" and ".css" files of theme design. This is necessary for easy update process.
For your own CSS-styles create block yourshop.user.css with content:
<style> /* Print here your CSS-styles */ </style>
For your own JS-scripts create block yourshop.user.js with content:
<script> /* Print here your JS-scripts */ </script>
When you update the theme design your changes will not be deleted.
Read more how to create blocks in the article.