How to change color of angular material stepper step icons

There does not seem to be option to change color of mat stepper icon, you can use this css as workaround.

 ::ng-deep .mat-step-header .mat-step-icon-selected {
    background-color: red; 
 }

::ng-deep is deprecated and can be removed, also can be used

ViewEncapsulation.None in component decorator to avoid using ::ng-deep

Update with solution to problem

html file example

 <div class="yellow-theme"> <----- wrapper theme class
     <button mat-raised-button (click)="isLinear = !isLinear" id="toggle- 
      linear">
            {{!isLinear ? 'Enable linear mode' : 'Disable linear mode'}}
          </button>
          <mat-horizontal-stepper [linear]="isLinear" #stepper>
            <mat-step [stepControl]="firstFormGroup">
              <form [formGroup]="firstFormGroup">
                <ng-template matStepLabel>Fill out your name</ng-template>
                <mat-form-field>
                  <input matInput placeholder="Last name, First name" 
                  formControlName="firstCtrl" required>
                </mat-form-field>
                <div>
                  <button mat-button matStepperNext>Next</button>
                </div>
              </form>
            </mat-step>
            <mat-step [stepControl]="secondFormGroup">
              <form [formGroup]="secondFormGroup">
                <ng-template matStepLabel>Fill out your address</ng-template>
                <mat-form-field>
                  <input matInput placeholder="Address" formControlName="secondCtrl" required>
                </mat-form-field>
                <div>
                  <button mat-button matStepperPrevious>Back</button>
                  <button mat-button matStepperNext>Next</button>
                </div>
              </form>
            </mat-step>
            <mat-step>
              <ng-template matStepLabel>Done</ng-template>
              You are now done.
              <div>
                <button mat-button matStepperPrevious>Back</button>
                <button mat-button (click)="stepper.reset()">Reset</button>
              </div>
            </mat-step>
          </mat-horizontal-stepper>

create theme.scss file and add it to styles in angular.json

 "styles": [
          "src/styles.css",
          "src/theme.scss"
           ]

note stepper will take color of primary color

theme.scss

 @import '~@angular/material/theming';
 @include mat-core();

 .yellow-theme {
     $yellow-theme-primary: mat-palette($mat-yellow, 400);
     $yellow-theme-accent:  mat-palette($mat-yellow, 400);

     $yellow-theme: mat-light-theme($yellow-theme-primary, $yellow-theme-accent);

     @include angular-material-theme($yellow-theme);
 }

Custom theme class can be used across application,just wrapp any material component and use color attribute primary,accent or warn as defined in class. Component that is wrapped in custom class wil use that color, if not color are set from global theme.