bug(dialog): low contrast for dialog backdrop in dark theme

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

Using Material 2 theme

### Description

Hi 👋,

While using the dark theme, I noticed that opening a dialog creates a readability issue due to the low contrast between the backdrop and the dialog content. Initially, I assumed this was a personal issue, but several team members have reported the same experience.

Upon reviewing the CSS code, I found that the backdrop color is [set to a single value](https://github.com/angular/components/blob/5ad133d07341fa8647e81277e7f1b9f54b15059a/src/cdk/overlay/_index.scss#L9), regardless of the theme. Currently, it's a faded black, which works fine for light themes but leads to poor contrast in dark themes. This reduces the legibility of the dialog content, making it harder to distinguish between the page background and the overlay.

In previous Material 2 theling API, dark themes used a faded white backdrop, which provided better contrast and enhanced readability.

### Reproduction

Steps to reproduce:
1. Go to https://m3.material.io/components/dialogs/specs and see that dialog example use a white backdrop on dark mode
2. Go to https://material.angular.io/components/dialog/overview, select a dark theme and open dialog in any example

### Expected Behavior


### Actual Behavior


### Environment

- Angular: 18.2.8
- CDK/Material: 18.2.8
- Browser(s): Chrome (latest)
- Operating System (e.g. Windows, macOS, Ubuntu): Ubuntu 24.10

src/cdk/overlay/_index.scss の backdrop 定義から始め、次に暗いテーマで Material dialog overview を使って問題を再現し、リンク先の Material 3 dialog example と比較します。暗いテーマでダイアログの backdrop のコントラストが読みやすく、明るいテーマの動作に悪影響を与えない状態になれば完了です。

索引モデルが issue の本文から書いたものです。