Angular Material2 theming - how to set app background?

Not exactly answer to your question, but I guess many people will end up here searching for "how to set app background color".

In your project/index.html set your body class to mat-app-background

<body class="mat-app-background">
  <app-root></app-root>
</body>

And make sure in your project/angular.json you have:

        "styles": [
          "./node_modules/@angular/material/prebuilt-themes/YOUR_STYLE.css",
          ...
        ],

If you want to change the theme's background color for the entire app in a clean way, you can override your theme with the following.

// Set custom background color
$custom-background-color: map_get($mat-blue-grey, 50);

// -or- Can set colour by hex value too
$custom-background-color: #628cc9;

$background: map-get($theme, background);
$background: map_merge($background, (background: $custom-background-color));
$theme: map_merge($theme, (background: $background));

This assumes you have already set up your $theme using mat-light-theme or mat-dark-theme. Of course you can substitute $mat-blue-grey for a color map of your choosing.

Here is a full example of how I am using this. I have the following in a file called theme.scss, which is included in my angular.json "styles" entry:

// Include the common styles for Angular Material. We include this here so that you only
// have to load a single css file for Angular Material in your app.
// Be sure that you only ever include this mixin once!
@include mat-core;

// Define the palettes for your theme using the Material Design palettes available in palette.scss
// (imported above). For each palette, you can optionally specify a default, lighter, and darker
// hue.
$primary: mat-palette($mat-red, 600, 400, 900);
$accent: mat-palette($mat-blue-grey, 500, 200, 700);
$background-color: map_get($mat-blue-grey, 50);

// The warn palette is optional (defaults to red).
$warn: mat-palette($mat-blue);

// Create the theme object (a Sass map containing all of the palettes).
$theme: mat-light-theme($primary, $accent, $warn);

// Insert custom background color
$background: map-get($theme, background);
$background: map_merge($background, (background: $background-color));
$theme: map_merge($theme, (background: $background));

// Include theme styles for core and each component used in your app.
// Alternatively, you can import and @include the theme mixins for each component
// that you are using.
@include angular-material-theme($theme);
@include my-app-theme($theme);