Merge polygons + set transparent boundaries

Hi

it would be great to merge polygons into “one” item.

To colour countries, I have imported GeoJSON files for each country, which worked like a charm.
(As suggested here and here.)

Unfortunately, the GeoJSON files of most countries consist of several items / polygons. So each of them shows up on the panel.

It would be great if several polygons could be merged into “one” country polygon item.