To use custom CSS and put it in your theme to override certain aspects of widgets or the storefront. Once you have the code or when you are ready to write the necessary code, you should customise your theme by navigating to the Theme editor.
In your Shopify dashboard,
Navigate to Sales Channels > Online Store > Themes
Click the Customize button.
On the Theme Editor page:
Go to the Theme settings — gear icon — in the left sidebar.
Scroll down to locate the Custom CSS section.
You can put or write the code in the text area field.
Once you are done, you can check the preview and hit Save.
