Adding CSS to Shopify themes - Alternative (but Not Recommended) Method
Although editing your Shopify theme is an alternative method for applying CSS, it is no longer the recommended approach. This method allows you to keep your styles within your theme, but it may require reapplication after theme updates.
For the recommended approach to adding custom CSS, please refer to the guide: How to Add Custom CSS the Right Way.
Adding Custom Code Using Custom Liquid Section
From your Shopify dashboard, go to Online Store > Themes > Select your current theme > Customize
Then select your sign customiser product or page template.
Choose to add a Custom Liquid section. The section name may vary in different themes.
Paste your custom code into the Liquid section. CSS code needs to be wrapped in a
<style></style>
tag, while JavaScript code needs a<script></script>
tag.
Some themes don't provide a custom Liquid section, so you may need to add the custom code directly to your theme. However, the code will not remain after theme updates.