bug(stepper): bad contrast of step icon in untouched state for M2

### Is this a regression?

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

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

_No response_

### Description

When using a custom theme, in the stepper it can happen that the step icon (step number or state icon) has a bad contrast:


It depends on the contrast color of the primary color. If it's white, everything works correctly, but if it's black, this issue occurs. However for the untouched state, it should always be white, regardless of the contrast color.

Dark mode has the same issue. Here it can even be reproduced in the v16 docs app after selecting the "Purple & Green" theme:


I was not able to reproduce this with M3, the predefined themes work fine. I didn't try custom M3 themes though.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/61i1fz?file=src%2Fstyles.scss
Steps to reproduce:
1. Configure a primary color with black as a contrast color in light theme, or white as a contrast color in dark theme
2. Use the stepper

### Expected Behavior

The step icon has sufficient contrast in untouched state

### Actual Behavior

The step icon has insufficient contrast in untouched state

### Environment

Angular CLI: 18.0.2
Node: 18.20.3
Package Manager: yarn 1.22.19
OS: linux x64

Angular: 18.0.1
... animations, common, compiler, compiler-cli, core, forms
... localize, platform-browser, platform-browser-dynamic, router

Package Version
------------------------------------------------------------
@angular-devkit/architect 0.1800.2
@angular-devkit/build-angular 18.0.2
@angular-devkit/core 18.0.2
@angular-devkit/schematics 18.0.2
@angular/cdk 18.1.2
@angular/cli 18.0.2
@angular/material 18.1.2
@angular/material-experimental 18.1.2
@angular/material-moment-adapter 18.1.2
@schematics/angular 18.0.2
rxjs 7.4.0
typescript 5.4.5
zone.js 0.14.6

リンクされた StackBlitz の再現例とその src/styles.scss から始め、説明されている明るいカスタムテーマと暗いカスタムテーマで stepper を使用します。主要なコントラストカラー全体で M2 untouched-state のアイコンのコントラストを比較します。両方の構成で step アイコンに十分なコントラストがあれば完了です。

索引モデルが issue の本文から書いたものです。