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. Click on the Assets heading on the left side to reveal your Assets content.
4. Your theme's main stylesheet will be located within this folder. Some common names for this file are:
Once you've found it, navigate to the bottom of the document and paste your CSS code.