Hi @Sally Oh34,
Tt's not a good idea to change fonts in your emails to something super custom, but you CAN change the fonts! Here's an article explaining how to do it and why Convertkit limits the number of fonts: https://help.convertkit.com/en/articles/4259989-using-custom-fonts-in-emails203
About the subscribe link... can you explain what you're trying to achieve? You can create a landing page where new people can sign up to get your emails, but people who you're already sending emails to are already subscribed, so a button to subscribe isn't necessary. You could include links in your emails that trigger tags, such as interests? Here's more on that: https://help.convertkit.com/en/articles/2502518-how-to-use-link-triggers27