How to theme FusionAuth's advanced registration forms

The tutorial provides an overview of customizing a theme for an advanced registration form using FusionAuth, a self-service authentication server. The process involves creating a new theme, modifying the messages file to add placeholders and labels, updating the theme in FusionAuth using the API, and previewing the changes. The tutorial also covers how to customize field display by modifying the `customField` macro and adding labels to each field. Additionally, it highlights the importance of reviewing themes in an incognito window for a better user experience.