How can I change elements which Design Ideas inserts in PowerPoint?

I really like PowerPoint's Design Ideas. I usually select one and then tinker with it a bit to get the look I like. However, I've noticed there are some elements I can't seem to touch. For example, in the image below, I can seemingly do nothing to the thin blue line between the two textboxes.

What if I don't want a thin blue line? What if I want a thick one? I've selected Ctrl+A in the image, so the line is not accessible by selecting all elements on the slide.

Any ideas?

Update: I've managed to access the Selection Pane, and through that found the "Straight Connector" which describes the thin blue line. Weirdly enough, I can format it to change its colour and shape, but I can't move it or directly access it. When I click on any of the other objects in the Selection Pane, the edges show around them and I can manipulate them as you would any object in PowerPoint.

Is this a bug, or is there a way around it? I'm using the Insider program version of Office, so it may be buggy (Selection Pane crashed my PowerPoint twice.)

It's not a bug, though certainly quite a few people have been unhappy with the feature's design for just this reason. MS is aware of that and who knows, they may change it at some point. If I'm not mistaken, the shapes in question are locked via a setting in the XML within your PPTX. Other than editing the XML, there's no way to change it.

You might want to fake a copy of the MS-provided design so that you've got something you can edit. For example, create a slide with the design you want, save it as an image, set the image as the background to a new slide, then draw atop it to replicate each shape from the original design.

Or maybe (and I haven't tried this) save the slide as an EMF, use Insert | Picture to bring the EMF onto a new blank slide, then ungroup the EMF (keep ungrouping until you can select the individual graphic components).