Can't seem to find the way to change the thin border color for a Windows UWP app. Many places point to this discussion: Change Accent Color in Windows 10 UWP. And I've used that information and other articles to change the SystemAccentColor, which changes some things about the running app. But after trying many different other magic theme keys to affect the window border I'm not sure what to do. Here's a picture of what I'm talking about - notice the magenta border along the edge.
You can't, this color is determined by the system accent color in the Settings app. You can only change the title bar color for your app.
(Have you seen any store app which was able to change this color?)