White borders are back in 1.17

> This problem is back in v1.17, I guess due to the recent changes around theming. The grey border is there to start with until you switch to another window and back (presumably it's part of the dark theme and doesn't get repainted until switching). Once you switch back it is white and will stay so even after closing/re-opening.
>
> Steps to reproduce:
> * Delete settings.json and state.json
> * Open Terminal and switch to light theme
> * Border is grey
> * Alt tab to another window then back again
> * Border is now forever white

_Originally posted by @harristom in https://github.com/microsoft/terminal/issues/6620#issuecomment-1406570765_

Start with the theming and window-border handling for Windows Terminal, then reproduce the issue by deleting settings.json and state.json, switching to the light theme, and Alt-tabbing away and back. Done means the border remains grey after returning to the window and after closing and reopening it; the issue names no source file or test.

Written by the indexing model from the issue text.