Changing border color in mat-form-field

For the newer outlined form fields, solved it this way:

::ng-deep {
     .mat-form-field-appearance-outline .mat-form-field-outline {
        color: white;
     }
     mat-form-field {
        .mat-hint, input, ::placeholder, .mat-form-field-label {
           color: white;
        }
     }
  }

Add this CSS to your form-field-appearance-example.css:

/* Border */
.mat-form-field-appearance-outline .mat-form-field-outline {
  color: white;
}
/* Font color */
input.mat-input-element {
  color: white;
}

Though, there is still a problem while the field is focused.

SCSS version of @sasa with colors as variables

::ng-deep {
   $custom-main-color: red;
   $custom-label-color: rgba(0, 0, 0, .6);

   // mat-icon-stepper selected color change
   .mat-step-header .mat-step-icon-selected, .mat-step-header .mat-step-icon-state-done, .mat-step-header .mat-step-icon-state-edit {
      background-color: $custom-main-color !important;
   }

   // input outline color
   .mat-form-field-appearance-outline .mat-form-field-outline {
      color: $custom-main-color !important;
   }

   // mat-input focused color
   .mat-form-field-appearance-outline.mat-focused .mat-form-field-outline-thick {
      color: $custom-main-color !important;
   }

   // mat-input error outline color
   .mat-form-field-appearance-outline.mat-form-field-invalid.mat-form-field-invalid .mat-form-field-outline-thick {
      color: $custom-main-color !important;
      opacity: 0.8 !important;
   }

   // mat-input caret color
   .mat-input-element {
      caret-color: $custom-main-color !important;
   }

   // mat-input error caret color
   .mat-form-field-invalid .mat-input-element, .mat-warn .mat-input-element {
      caret-color: $custom-main-color !important;
   }

   // mat-label normal state style
   .mat-form-field-label {
      color: $custom-label-color !important;
   }

   // mat-label focused style
   .mat-form-field.mat-focused .mat-form-field-label {
      color: $custom-main-color !important;
   }

   // mat-label error style
   .mat-form-field.mat-form-field-invalid .mat-form-field-label {
      color: $custom-main-color !important;
   }
}

I think this will cover everything.

// mat-icon-stepper selected color change 
::ng-deep .mat-step-header .mat-step-icon-selected, .mat-step-header .mat-step-icon-state-done, .mat-step-header .mat-step-icon-state-edit {
    background-color: red!important;
}

//input outline color
::ng-deep .mat-form-field-appearance-outline .mat-form-field-outline {
    color: red!important;
    // opacity: 1!important;
}

//mat-input focused color
::ng-deep .mat-form-field-appearance-outline.mat-focused .mat-form-field-outline-thick {
    color: red!important;
}

// mat-input error outline color
::ng-deep .mat-form-field-appearance-outline.mat-form-field-invalid.mat-form-field-invalid .mat-form-field-outline-thick{
    color: red!important;
    opacity: 0.8!important;
}

// mat-input carent color
::ng-deep .mat-input-element {
    caret-color: red!important;
}

// mat-input error carent color
::ng-deep .mat-form-field-invalid .mat-input-element, .mat-warn .mat-input-element {
    caret-color: red!important;
}

// mat-label normal state style
::ng-deep .mat-form-field-label {
    color: rgba(0,0,0,.6)!important;
    // color: $mainColor!important;
}

// mat-label focused style
::ng-deep .mat-form-field.mat-focused .mat-form-field-label {
    color: red!important;
}

// mat-label error style
::ng-deep .mat-form-field.mat-form-field-invalid .mat-form-field-label {
    color: red!important;
}