Hi,
When I open iMovie 10.1.8 (the latest version) and click on Window/Theme Chooser, I get a screen showing all of the themes, including the News theme. There are 15 of them. Are you saying that when you do that you don't get the themes, or get them all except the News theme?
If so, try opening iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box that appears. Then see if you can get the themes following the procedure I outlined above.
If still no luck, you may have gotten a bad download of iMovie, because the themes should be there. In that case, try redownloading iMovie 10.1.8. Also update to High Sierra if you haven't already done so.
-- Rich