Schematic does not add custom theme

[jerryorta-dev]

Bug, feature request, or proposal:

When creating a new angular project using version 6.0 fo @angular/cli,
And adding material with a custom theme using the schematic -- ng add @angular/material --theme=custom,
Then a default custom theme should be applied to the file src/styles.scss.

What is the expected behavior?

The stubbed custom theme contents should be added to the file src/styles.scss.

What is the current behavior?

The follow error is thrown in the console: "No style files could be found into which a theme could be added"

What are the steps to reproduce?

In the console:

# create a new project with scss as default stylesheet
ng new test-styles --style=scss

# change directories
cd test-styles

# Use schematic to add material with custom theme
ng add @angular/material --theme=custom

What is the use-case or motivation for changing an existing behavior?

To add a custom theme to my projects.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Node 8.10.0
Angular 6.0.0
Material 6.0.0

Is there anything else we should know?

As a work-around, before adding material, in the angular.json file, change project “styles” to

"styles": [
  {
    "input": "src/styles.scss"
  }
], 

And then it works fine.

The issue appears to be here, which requires src/styles.scss to in an object.

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