Setting and removing `window.activeBorder` color does not reset the window border color

[benibenj]

Testing #263838

1. window.border: default

"workbench.colorCustomizations": {
    "window.activeBorder": "#ff00ff",
}

3. Window color color changes to pink
4. Remove "window.activeBorder" from workbench.colorCustomizations
5. 🐍 window stays pink instead of using theme color