Minor problems with building inside Unreal.

Just a few quick questions. The first issue is I can’t seem to get these floor tiles I made to click together? Or more precisely, just slide together so their is no gap between each tile.

And the second question is if I was to build everything (im trying to create a bedroom) how would I change the entire atmosphere to pitch black? Like everything in unreal starts out with sky background, how would I change the background to a pitch black. (Because this is only the first level) Thank you in advance!!