When you add the Snap-ins component to your Community page, you get this window.
I see that you can add External Scripts and External Styles. Is it possible to add scripts and styles that customize the snap0ins chat code? Like adding a pre-chat background image url, or passing nonstandard pre-chat details?
As far as I understand, when reading https://resources.docs.salesforce.com/214/latest/en-us/sfdc/pdf/snapins_web_dev_guide.pdf, this is only used for sending custom events back and forth from agent to customer and vice versa. I'm sure you might be able to add some extra things in there, but not sure if they will work or not.