Follow up from #994 and PR:#3060

In the aforementioned PR, we're hardcoding the border width to 2. This should really be configurable to any positive double.

Questions:

• (Similar to the discussion in Add a setting to manually set the Pane highlight color #3061) 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?
  • If we do want to do this in XAML later, would enabling it in the json now negatively impact that future scenario?