Theme specifies the color of the components, the darkness of the surfaces, level of shadow, appropriate opacity of ink elements, etc. You can either create your own Theme or use Themes that provide from refine. Theme provides a way to your app's design to meet them.
📄️Migration Guide from v4 to v5
Material UI released a new major version for @mui/x-data-grid package as v6. This document will guide you through the migration process for the refine related parts. While this document won't cover all the changes, it will help you get started with the migration process.