Material button not displaying properly despite adding theme

Question

I am using material buttons in my angular project

   <button mat-button mat-dialog-close>Buy</button>
   <button mat-button mat-dialog-close>Cancel</button>

and added theme in custom-theme.scss

   @import '~@angular/material/prebuilt-themes/indigo-pink.css';

but only normal HTML buttons got rendered, instead of mat button.

It's happening similarly for some other components as well (mat-form-field). Any thoughts?