Hello @baohuachu2 !
Calcite is Esri's design system that ArcGIS web-based experiences use (except for Experience Builder). We continue to invest heavily in it as build new components, enhance existing ones, add guide topics, and do overall improvements across the design system (for example, we recently worked on better responsive design and design tokens, etc).
The jimu application framework is specific to Experience Builder. It is not based on Calcite because of the development timeframes (Calcite components weren't available when ExB was being built/released). Many customers have requested that Esri creates a Calcite-based theme within ExB, which would blend well with components coming from the JavaScript Maps SDK (also based on Calcite) and any custom ExB widgets that are built with calcite components. This is planned, but we don't have a release date at this time (to my knowledge - perhaps @Jianxia has a more up to date answer on that).
That said, even though we don't have a Calcite theme in ExB yet, you can use Calcite to build custom widgets today. You can find doc on that here --> https://developers.arcgis.com/experience-builder/sample-code/widgets/use-calcite-components/
I hope that helps!
