Clicking the skydome in the atmosphere section does in fact do something! It creates a tiiiiny skydome at 0,0. Large enough for the world's tiniest mod.
None of the main campaign levels seem to use skydomes, so I don't think it's a feature they finished implementing. If you want a backdrop for your menu or character creation camera, you can search root templates for "dome", but there's only 3 or so.
