I have created a custom theme(Material 2) for my angular 4.x app. I have imported it in my Css file but getting error cannot get Theme.scss

Theme.scss

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

$my-app-primary: mat-palette($mat-blue-grey);
$my-app-accent:  mat-palette($mat-pink, 500, 900, A100);
$my-app-warn:    mat-palette($mat-deep-orange);

$my-app-theme: mat-light-theme($my-app-primary, $my-app-accent, $my-app-warn);

@include angular-material-theme($my-app-theme);

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

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

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

How I imported in CSS file

@import "../node_modules/@angular/material/prebuilt-themes/Theme.scss";

I have saved my custom theme inside prebuilt themes folder .The problem is with custom themes only if I import prebuilt css theme file it's working