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.

Of course, you can use your own CSS and customize the background & foreground color. There's no need for ::ng-deep. (/deep/ and ::ng-deep are both currently deprecated)

With a couple of SCSS here's how my stepper looked: I have added SASS snippets needed for customizing the label and icons below:

Completed step & its label:

.mat-step-header .mat-step-icon-state-done, .mat-step-header .mat-step-icon-state-edit {
  background-color: transparent !important;
  color: $success;
  +.mat-step-label{
    color: $success !important;
  }
}

In Progress step & its label:

.mat-step-header .mat-step-icon-selected{
  // background-color: $primary;
  background-color: transparent !important;
  color: $primary;
  +.mat-step-label{
    color: $primary !important;
  }
}

Todo/Default step & its label:

.mat-step-header .mat-step-label{
    color: rgba(0, 0, 0, .54)
}
.mat-step-header .mat-step-icon {
    color: rgba(0, 0, 0, .54);
    background-color: transparent;
}

HTML for overriding default icons: I used font awesome icons to override the defaults:

<mat-horizontal-stepper>
    <!-- Icon overrides. -->
    <ng-template matStepperIcon="edit">
        <i class="fa fa-check-circle"></i>
    </ng-template>
    <ng-template matStepperIcon="active">
        <i class="fa fa-dot-circle-o"></i>
    </ng-template>
    <ng-template matStepperIcon="done">
        <i class="fa fa-dot-circle-o"></i>
    </ng-template>
    <ng-template matStepperIcon="number">
        <i class="fa fa-dot-circle-o"></i>
    </ng-template>
</mat-horizontal-stepper>

Note: All SCSS should be in the global stylesheet and not in the component specific stylesheet. If the styles are present inside the angular component's file it will not be applied due to view encapsulation.

Please define the color variables if needed at the start of the SCSS file as below: Replace the HEX values with your theme colors.

$success:       #35A255 !default;
$primary:       #007CBB !default;

Using Ionic v4, in my case just added (inside :root):

  .mat-step-header .mat-step-icon-selected {
    background-color: var(--ion-color-primary);
  }

To variables.scss file.

Inside of your scss file for you component do the following:

::ng-deep {
    .mat-focused .mat-form-field-label {
      color: green;
    }

    .mat-form-field-underline {
      background-color: green;
    }

    .mat-form-field-ripple {
      background-color: green;
    }
 }

Why you should use ng-deep instead of ViewEncapsulation.none

Disabling effect of ViewEncapsulation.None in Angular

https://medium.com/ng-gotchas/piercing-the-angular-style-encapsulation-c7030caeb519