Add CSS to your theme
1. From your Shopify admin, click Online Store, then click Themes.
2. Find the theme you want to edit, click the Actions button, then click Edit code.
3. On the left side, click on the Assets heading to reveal the folder's content.
4. Your theme's main stylesheet will be located within this folder. Some common names for this file are:
5. Once you've found it, navigate to the bottom of the document and paste your CSS code.