Follow up from #994 and PR:#3060

In the aforementioned PR, we're just using the accent color for highlighting the pane. We should let that be a customizable global setting.

Questions:

• There's been discussion before about allowing custom theming, hypothetically powered by XAML. Would we want to wait for that discussion to be had before we do this?
  • We'd probably want to change it from using SystemAccentColor to some FocusedPaneBorder color resource, that we define by default to be SystemAccentColor, should we decide to pursue this option
  • If we do want to do this in XAML later, would enabling it in the json now negatively impact that future scenario?