Migrate Customizations to Agent Host Harnesses: make UI consistent with prompt migration

Testing https://github.com/microsoft/vscode/issues/330793

- enable setting "chat.customizations.userDataMigration.enabled": true,
- run Chat: New Instructions file, select User Data, enter a name 'test'. A file `test.instructions.md` is created in user data
- in a VS Code window create a new chat, select Copilot as harness
- open the customizations dialog 'Chat: Open Customizations`
- select the `Migrate User Data` tab, and compare with `Migrate Prompts`

`Migrate User Data` uses a box with a background color which I find a bit too heavy.
They should be consistent