I would do this by overriding a CSS variable using renderer2:

// styles.scss

:host {
  --app-font-size: 16px;
}

body {
  font-size: var(--app-font-size);
}

The updateFontSize method in the example below takes an explicit size argument, but you could do a mathematical increment/decrement calculation instead.

// font-size.component.ts

export class FontSizeComponent {
  constructor(private renderer: Renderer2) {}

  updateFontSize(size: string) {
    this.renderer.setProperty(
      document.documentElement,
      'style',
      `--app-font-size: ${size}`
    );
  }
}

<!-- font-size.component.html -->

<button (click)="updateFontSize('18px')">+ Font Size</button>
<button (click)="updateFontSize('14px')">- Font Size</button>