Toggle button seems not have css - css

Question

I need to create a dropdown button, so I installed bootstrap and ng-boostrap. This is my html:
<div ngbDropdown class="d-inline-block">
<button class="btn btn-outline-primary" id="dropdownBasic1" ngbDropdownToggle>Toggle dropdown</button>
<div ngbDropdownMenu aria-labelledby="dropdownBasic1">
<button class="dropdown-item">Action - 1</button>
<button class="dropdown-item">Another Action</button>
<button class="dropdown-item">Something else is here</button>
</div>
</div>
and in my app.module.ts I put:
imports:{
NgbModule.forRoot()}
I have just also put bootstrap css in my angular.json file.
The problem is that dropdown appear grey like it doesn't have css. I don't know what is the problem. Anyone can help me?
EDIT
My angular.json file have this:
"styles": [
"src/styles.css",
"node_modules/bootstrap/dist/css/bootstrap.min.css",
"node_modules/font-awesome/css/font-awesome.css"
],

Answer 1

Step 1) install #ng-bootstrap/ng-bootstrap
npm install --save #ng-bootstrap/ng-bootstrap
Step 2) Import the ng-bootstrap package in app.module.ts file
import {NgbModule} from '#ng-bootstrap/ng-bootstrap';
#NgModule({
...
imports: [NgbModule, ...],
...
})
export class AppModule {
}
Note: No need of doing NgbModule.forRoot()
Step 3) Use your html code in app.component.html file
<div ngbDropdown class="d-inline-block">
<button class="btn btn-outline-primary" id="dropdownBasic1" ngbDropdownToggle>Toggle dropdown</button>
<div ngbDropdownMenu aria-labelledby="dropdownBasic1">
<button class="dropdown-item">Action - 1</button>
<button class="dropdown-item">Another Action</button>
<button class="dropdown-item">Something else is here</button>
</div>
</div>
I have created the same example, which is forked from the ng-bootstrap.
View Demo
Hope this will help you.

Answer 2

I solved this problem by adding line:
#import '~bootstrap/dist/css/bootstrap.min.css'
to my /src/styles.css file.
(You need have bootstrap or bootstrap-css-only installed. To install run command npm install bootstrap-css-only --save)