Hi @Dan Kirk , to add custom branding per domain you can follow this document.
"You can customize your Azure AD sign-in pages, which appear when users sign in to your organization's tenant-specific apps, such as https://outlook.com/contoso.com, or when passing a domain variable, such as https://passwordreset.microsoftonline.com/?whr=contoso.com."
Please let me know if you have any questions or if I can help you further. If this answer helped you please mark it as "Verified" so other users can reference it.
Thank you,
James