Terrific sample project, by the way.
According to https://material.angular.io/guide/theming
The theme file should not be imported into other SCSS files. This will cause duplicate styles to be written into your CSS output. If you want to consume the theme definition object (e.g., $candy-app-theme) in other SCSS files, then the definition of the theme object should be broken into its own file, separate from the inclusion of the mat-core and angular-material-theme mixins.
I'm (very) new to scss and Angular Material, so I may be miss-interpreting this, but where you
@import '../../styles/shared/_colors';
eg in features.component.scss
isn't this doing just what the above tells us not to do, as mat-core is included in _colors.scss?