You can customise the login screen for your business/family on the Manage → Customise Login screen.
To make customisation quick and easy, you only need to specify 3 things:
By default, the custom login screen will then appear at http://mail.yourdomain.com.
If you host DNS for your domain with us, this will just work automatically. If you use an external DNS provider, you’ll need to create a CNAME record for mail.yourdomain.com that points to www.fastmail.com.
An example page is viewable at http://mail.digitalintegrity.com.
For resellers, any sub-business will automatically get your business customised login screen by default, though you can also explicitly customise the login screen for each sub-business if you want.