Adopted Style Sheets You can use the adoptStyleSheet() function to add a CSSStyleSheet instance to a component's list of adopted style sheets. The css tagged template function will construct a CSSStyleSheet object that can be adopted by documents and shadow roots. import { customElement, css } from 'thunderous'; const myStyleSheet = css` :host { display: block; font-family: sans-serif; } `; const MyElement = customElement(({ adoptStyleSheet }) => { adoptStyleSheet(myStyleSheet); // ... }); When opting out of shadow DOM, the adoptStyleSheet() will use the global document to adopt the styles, and :host selectors will have no effect. Learn how to opt out of shadow DOM.