Title bar background is applied to entire window in experimental Modern UI

Does this issue occur when all extensions are disabled?: Yes.

- VS Code Version: 1.134.0 (system setup)
- OS Version: Windows 11, 26200.9168

Steps to Reproduce:

1. Apply the `titleBar.activeBackground` setting.
2. Enable the Modern UI. (This was enabled by default for me.)

Context: we tend to have multiple VSCode windows open for various projects. To disambiguate windows more easily, we color-code our projects using `workbench.colorCustomizations`, specifically the subkeys `titleBar.activeBackground` and `titleBar.inactiveBackground`. The new modern UI (which I generally dislike anyway) really makes the whole editor window more ugly now.

For example, project 1:

And project 2:

Frankly, it makes my _code editor_ look like a circus. This is really not acceptable. If you truly must force this modern UI update, at least use a different color theme key for the workbench background.