Add themes as any other module. See the demo app in the repository for Highcharts Angular wrapper to see a code example used there.

A short load and initialization example is below:

import * as Highcharts from 'highcharts'; // load core

import SunsetTheme from 'highcharts/themes/sunset.src.js'; // load a theme

SunsetTheme(Highcharts); // init the theme

More info about custom themes in Highcharts general documentation.