I added a new color id 'editor.rangeHighlightBorder' that controls the border color you mention.
No change of out-of-the box behaviour. It's only set in highContrast and it is the contrastActiveBorder color by default (you can change it...)

The name seems suboptimal as it sounds similar to editor.findRangeHighlightBackground and would suggest it only affects the same text. We're talking about at least 3 things here whose background colors are editor.findMatchBackground, editor.findMatchHighlightBackground and editor.findRangeHighlightBackground.
If individual border colors were created for all 3, then there would be an editor.findRangeHighlightBorder which sounds very similar to your name. If indeed your new color affects all 3, it should have a more generic name (such as editor.contrastActiveTexBorder).

Just to make sure we're on the same page. Aside from the findRange... there are other occurrences, such as peekViewResult.*
So prefixing the name with "editor." would not be wise. I think my original suggestion contrastActiveTextBorder would work really well as a generic name and it would be easy to notice in the suggestions box while editing the user settings, because it would pop up while you're entering "contrastActive..." to set the contrastActiveBorder.

Good point, there are more uses of contrastActiveBorder that I missed. I added more border settings as having border settings for the decorations was a request from others as well.

Agree that range highlight and 'findRange highlight' 'findMatchRange highlight' might be confusing, but I want to stick with the terminology that we started.

The border around the identifier name in "Peek Definition" is still not themeable.

Other than that things look good. I can activate contrastActiveBorder in my theme now without getting eye cancer. Thank you very much.