app.js

import { css, html, LitElement } from 'lit-element';
import { foo } from './styles.js';

customElements.define('fancy-app', class extends LitElement {
  
  static get styles() { return [
    foo,
    css`
      .bar { color: pink; }
    `
  ]};
  
  render() {
    return html`<div class="bar">Wowowowia</div>`;
  }
});

styles.js

import { css } from 'lit-element';

export const foo css`.bar { background: background };`;