White border when Terminal is dark while Windows is in Light theme

[hylowaker]

Environment

Windows build number: Microsoft Windows [Version 10.0.19582.1001]
Windows Terminal version (if applicable): 0.10.761.0

Steps to reproduce

1. Set Windows theme to Light.
2. In profiles.json, set "requestedTheme": "dark" and "showTabsInTitlebar": true
3. Focus on the window.

Expected behavior

No white borders, regardless of Windows theme color. (Like as VSCode)

Actual behavior

Though #4817 fixes the white border issue for Dark mode in Windows, the border still presents with the combination of dark theme for Terminal settings and light theme for Windows.

[DHowett-MSFT]

So, we're doing our level best with the system API here. I'm going to keep this one around tracking our switch to the final API when it's out.