Shopify has 4 plans: Basic, Shopify, Advanced, and Plus. Officially, Shopify only allows changing the Checkout page on the Plus plan (subscription $2000/month). However, Shopify provides the ability to embed your own script on the Checkout page. This feature can be used to modify the Checkout page: you can change elements on the page as well as alter the appearance using styles.
Your code is embedded in the store admin area under Online Store / Preferences / Additional Google Analytics Javascript.
