Here's a very descriptive guide on how to define your own theme (but you should definitely check out the link that @br.julien posted above.):

NOTE: If you're in a rush, just scroll down to the bottom of this answer to get the whole source code.

Prerequsites

• If you're using a CSS file, please change the filename extension to .scss.

• If you're using the Angular CLI, in your styles array, change styles.css to styles.scss:

  "styles": [
      "styles.scss" // <- Change to this
  ]
  

Get started

1. Import ~@angular/material/theming at the top of your styles.scss:

   @import '~@angular/material/theming';
   

2. After that, add @include mat-core() in styles.scss, ideally after the import line:

   @include mat-core();
   

3. Afterwards, define some variables for your app (primary, accent and optionally warn), then assign them to a function called mat-palette:

   // I suggest that you should use another prefix, but `my-app` is fine as well
   // Primary colour
   $my-app-primary: mat-palette($mat-indigo);
   // Accent colour
   $my-app-accent: mat-palette($mat-pink, A200, A100, A400);
   

   For all palettes, visit the source code (_palette.scss).

   Also, see what the colour palettes look like on Material.io guidelines.

4. Next, define another variable for your app's theme and include the theme as a parameter of angular-material-theme (place this after the variables you defined above):

   (Note: Choose any one of these options below:)

   Light theme:

   $my-app-theme: mat-light-theme($my-app-primary, $my-app-accent);
   @include angular-material-theme($my-app-theme);
   

   Dark theme:

   $my-app-theme: mat-dark-theme($my-app-primary, $my-app-accent);
   @include angular-material-theme($my-app-theme);
   

5. You're done!

Full source code

Here's some full source code for you to copy :)

styles.scss

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

$my-app-primary: mat-palette($mat-indigo);
$my-app-accent: mat-palette($mat-pink, A200, A100, A400);
$my-app-theme: mat-light-theme($my-app-primary, $my-app-accent);

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

(Wow. All this work for just 8 lines of code.)

Anyways, if you want to learn more about Sass, check out the official documentation!