Marking this resolved as it has been fixed in 4.1. See details here: https://confluence.atlassian.com/servicedesk/jira-service-desk-4-1-x-release-notes-963663641.html#JiraServiceDesk4.1.xreleasenotes-newportalMeetthenewportalexperience
Can also check out the blog (https://confluence.atlassian.com/servicedesk/a-stylish-new-portal-for-jira-service-desk-server-and-data-center-customers-963663645.html#AstylishnewportalforJiraServiceDeskServerandDataCentercustomers-loginCustomizableloginscreen).
We are aware that this is important and something our customers expect. The good news is that, this is a priority in our short term roadmap and we're currently investigating ways to make this available to our customers.
Whilst I’d like to give you a clear timeframe for when we can deliver this, unfortunately, I cannot make such indications at this time yet. We’ll keep you posted as we progress.
Thanks again for your patience,
Product Manager - Jira Service Desk.