Angular project from stackblitz - no Bootstrap styles

Question

I have downloaded the following project from StackBlitz:

Example

I had to modify app.module by adding this code:

import {APP_BASE_HREF} from '@angular/common';

and this:

providers: [{provide: APP_BASE_HREF, useValue: ''}],

After that it ran, but Bootstrap 4 is not taken into account. What might be wrong?

Thanks