I know you can use CSS, but where do you put the CSS code?

Not having much luck finding the answer despite my digging.

Thanks in advance!

you can use css in 'Edit Description' - either via the menu by going Pet >> Pet (under the pet whose profile you want to change) >> Edit Description or in the down chevron ⌄ menu for that pet in the main Pets page.

Thank you! I was making that way more complicated in my head than it actually was. 😂

There is a navigation menu on the top right of your screen. Personal > Pets On this page, each pet has a square with their information. In the top right of this square is a V which is a drop down menu. Click on this and you'll see "edit description". The edit description page is where you can put in custom CSS or HTML, your story, etc.