I want to use GeoJson Data Source to load geojson to draw national boundaries, but to make the boundaries dashed.
You would need to load the GeoJSON, then iterate over the entities created in CesiumJS and modify their properties.
This code example does something similar, but extrudes the polygons instead: https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=GeoJSON%20and%20TopoJSON.html