Check out example codes for "add css dynamically in angular 6". It will help you in understanding the concepts better.

Code Example 1

import { CssService} from './Css.service';

@Component({
  selector: 'DynamicCss',
  templateUrl: './DynamicCss.component.html',
  styleUrls: ['./DynamicCss.component.scss']
})
export class ServiceProviderComponent implements OnInit {
    cssVariables: any;
    constructor(private cssService:CssService){
        /* call the service/api to get the css variable values in cssVariables */

    }
}

Code Example 2

ngOnInit(){
 this.appendCss(this.customizeFormData);
}

Code Example 3

appendCss(customData)
{
     let text = '.custom-form-1 {
            background-image: url("`+customData.background_image+`");
         }';
     $(document).ready(function(){
         $("style").append(text);
      });
}

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