Currently, the Jira Service Desk customer portal landing page form can't be customized using CSS. Administrators expect to have similar template as Contact Support page ( https://support.atlassian.com/contact )
Provide an option for admin to customize the form.
There is a disconnect between the Atlassian support contact page, and the landing pages that are found within Jira Service Desk customer portals. Jira admins expect to have either own product to have the same or equivalent features that Atlassian's own support uses.