Custom CSS can be added to your entire theme that affects all pages in your online store except the Checkout page.
From your Shopify admin, go to Online Store -> Themes.
Find the theme that you want to edit, and then click Customize.
Click Theme Settings -> Custom CSS.
See Shopify Adding custom CSS to your themearrow-up-right here for more details about using this setting on your store.
Last updated 1 year ago